Method of manufacturing an acetabular component

ABSTRACT

An orthopedic component having multiple layers that are selected to provide an overall modulus that is substantially lower than the modulus of known orthopedic components to more closely approximate the modulus of the bone into which the orthopedic component is implanted. In one exemplary embodiment, the orthopedic component is an acetabular shell. For example, the acetabular shell may include an outer layer configured for securement to the natural acetabulum of a patient and an inner layer configured to receive an acetabular liner. The head of a femoral prosthesis articulates against the acetabular liner to replicate the function of a natural hip joint. Alternatively, the inner layer of the acetabular shell may act as an integral acetabular liner against which the head of the femoral prosthesis articulates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/362,159, filed on Jan. 29, 2009, which claims the benefit under Title35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No.61/104,870, entitled “ORTHOPEDICE COMPONENT OF LOW STIFFNESS”, filed onNov. 13, 2008, U.S. Provisional Patent Application Ser. No. 61/038,281,entitled “ORTHOPEDICE COMPONENT OF LOW STIFFNESS”, filed on Mar. 20,2008, U.S. Provisional Patent Application Ser. No. 61/024,737, entitled“ACETABULAR COMPONENT”, filed on Jan. 30, 2008, and U.S. ProvisionalPatent Application Ser. No. 61/024,778, entitled “ACETABULAR COMPONENT”,filed on Jan. 30, 2008, the entire disclosures of which are herebyexpressly incorporated by reference herein.

BACKGROUND

1. Field of the Invention

The present invention relates to orthopedics, and, particularly, toorthopedic implants.

2. Description of the Related Art

During orthopedic surgery, such as a total hip arthroplasty, orthopedicprostheses are implanted into a patient's body. For example, a femoralstem may be implanted into the intramedullary canal of the patient'sfemur. After the stem is inserted into the intramedullary canal at adesired depth and orientation, a femoral head may be attached to theproximal end of the stem. The femoral head fits into a substantiallyhemispherically shaped socket of an acetabular prosthesis and provides asurface for articulation between the femoral head and the acetabularprosthesis.

In certain embodiments, acetabular prostheses comprise two separatecomponents, an acetabular backing component and an acetabulararticulating component, which are often referred to as a shell andliner, respectively. The backing component is generally hemisphericallyshaped and is affixed and embedded into an acetabulum of a patient.Similarly, the liner has a hemispherical shape configured to mate withan internal cavity of the backing component. The liner further includesa socket to mate with a femoral head. The acetabular backing componentsare typically formed of solid metal, such as titanium or a titaniumalloy, or from stainless steel. In contrast, the acetabular articulatingcomponents may be formed from an ultra-high molecular weightpolyethylene polymer material (“UHMWPE”). These types of acetabular cupsare often referred to in the art, and sometimes herein, as “metalbacked” components.

Metal backed orthopedic components generally experience contact stressesduring normal joint articulation. For example, in conventional metalbacked acetabular components, previous studies have shown that polymerthickness and conformity between the femoral head and the polyethyleneliner play a significant role in the level of contact stressexperienced. For highly conforming acetabular component designs, thecontact stress in the polymer liner is very sensitive to the polymerthickness when the polymer thickness is small. Specifically, it has beendemonstrated that when the polymer thickness is less than 4-6 mm, thecontact stress generally increases rapidly as the thickness decreases.Therefore, a minimum polymer thickness of 4-6 mm has been maintained formetal backed acetabular components in an attempt to lessen the contactstress. Further, having a 4 mm minimum thickness of the polyethyleneliner in an acetabular component has been widely accepted as an industrydesign rule and has been traditionally relied upon in the manufacture ofacetabular components.

Stress shielding is a phenomenon that can potentially occur with metalbacked orthopedic components and can occur when a portion of the stressnormally exerted on a patient's bone is instead borne by the orthopediccomponent, such as an acetabular cup. As result, the bone may begin toundergo atrophy and decalcification resulting in a weaker bonestructure. In severe stress shielding, the bone may be resorbed by thebody, decreasing the amount of bone stock at the fracture site.

SUMMARY

The present invention, in one exemplary embodiment, provides anorthopedic component having multiple layers that are selected to providean overall modulus that is substantially lower than the modulus of knownorthopedic components to more closely approximate the modulus of thebone into which the orthopedic component is implanted. In one exemplaryembodiment, the orthopedic component is an acetabular shell. Forexample, the acetabular shell may include an outer layer configured forsecurement to the natural acetabulum of a patient and an inner layerconfigured to receive an acetabular liner. The head of a femoralprosthesis articulates against the acetabular liner to replicate thefunction of a natural hip joint. Alternatively, the inner layer of theacetabular shell may act as an integral acetabular liner against whichthe head of the femoral prosthesis articulates.

In one exemplary embodiment, the outer layer of the acetabular shell isformed from a metal and the inner layer is formed from a biocompatiblepolymer. For example, the outer layer may be formed from a porous metaland the inner layer from a polyaryletherketone (“PAEK”), such aspolyetheretherketone (“PEEK”), or from a UHMWPE, such as an antioxidantstabilized UHMWPE. To form an acetabular shell according to the presentinvention, the outer layer is formed as a cup-shaped body usingtraditional techniques. Once formed, the biocompatible polymer, such asPEEK or UHMWPE, is secured to the outer layer to form an inner layer,resulting in a substantially completed acetabular shell. In exemplaryembodiments, the polymer is attached to the outer layer by injection orcompression molding. In one exemplary embodiment, the orthopediccomponent is complete after attachment of the outer and inner layers,e.g., after injection or compression molding, and requires no furthermachining or modification prior to implantation. In another exemplaryembodiment, the orthopedic component may be further machined afterattachment of the outer and inner layers to provide the final details,dimensions, and/or features that are desired.

Attaching a biocompatible polymer to a porous material to form anorthopedic implant allows the polymer to be received within the pores ofthe material. When the polymer hardens, the interaction of the twomaterials provides a firm securement between the porous layer and thepolymer layer to form the orthopedic component. Additionally, when thebiocompatible polymer is compression or injection molded, for example,to the porous material, a locking feature, such a groove configured toreceive a corresponding snap ring, for example, may be integrally formedin and/or on the polymer. By integrally forming a locking feature intoor on the orthopedic component, the machining steps necessary to finishthe orthopedic component may be reduced and/or entirely eliminated.

Advantageously, by manufacturing an orthopedic component in accordancewith embodiments of the present invention, the orthopedic component hasa stiffness which is lower than the stiffness of other orthopediccomponents. As a result, the present invention lessens the effects ofstress shielding on the natural bone stock, reducing the potential forosteolysis and bone resorption.

Additionally, the present invention provides a surgeon with the abilityto drill bone screw receiving apertures into the orthopedic component atthe most advantageous locations. Thus, when implanting an acetabularshell, for example, the surgeon may identify the areas of the acetabulumhaving the greatest bone stock and then drill apertures through theacetabular shell which are aligned to utilize this bone stock. As aresult, each orthopedic component can be custom fit to the individualpatient to facilitate the optimal retention of the orthopedic componentby the natural bone stock. Additionally, allowing a surgeon to customizethe location of the bone screw receiving apertures reduces inventory andmachining costs by eliminating the need to manufacture and stockorthopedic components with various bone screw receiving apertureconfigurations. Further, the method of manufacture of the presentinvention may be used to create a smooth surface on the polymer of theorthopedic component against which another orthopedic component and/ornatural bone stock may articulate. By creating a smooth surface on thepolymer, the generation of wear debris may be lessened.

Further, by utilizing an antioxidant stabilized polymer, such as UHMWPEblended with vitamin E, the overall thickness of the polymer layer maybe lessened. Thus, the polymer layer may have a thickness of less then 4mm, for example, without demonstrating an unacceptable level of contactstress. This results in reduced material and manufacturing costs.Further, by utilizing an antioxidant stabilized polymer, long termoxidation of the polymer may be lessened, increasing the useful life ofthe orthopedic component.

In one form thereof, the present invention provides an orthopedic systemincluding: a substantially hemispherical outer layer formed from a firstporous material; and a substantially hemispherical inner layer formedfrom a polyaryletherketone, wherein the polyaryletherketone at leastpartially permeates the pores of the first porous material so that atleast a portion of the polyaryletherketone interdigitates with pores ofthe first porous material, the first porous material and thepolyaryletherketone cooperating to define an acetabular shell having aneffective stiffness between 0.1 GPa and 15 GPa.

In another form thereof, the present invention provides an acetabularcomponent configured for use in a hip replacement surgery, theacetabular component including: a porous layer configured to contact andinterface with bone tissue when the acetabular component is implanted;an inner layer formed from an antioxidant stabilized, crosslinkedultrahigh molecular weight polyethylene and having a thickness of lessthan six millimeters, the inner layer configured to receive a femoralcomponent; and an interdigitation layer defined by the distance overwhich the antioxidant stabilized, crosslinked ultrahigh molecular weightpolyethylene of the inner layer infiltrates pores of the porous layer.

In yet another form thereof, the present invention provides a method ofmanufacturing an orthopedic component for implantation into a bone, theorthopedic implant having a bone contacting layer, an interdigitationlayer, and an inner layer, the method comprising the steps of:determining the elastic modulus of the bone; selecting a thickness of atleast one of the bone contacting layer, an interdigitation layer, and aninner layer based on the elastic modulus of the bone; and molding theinner layer to the bone contacting layer to form at least one of thebone contacting layer, an interdigitation layer, and an inner layer tohave the selected thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is cross-sectional view of an exemplary orthopedic implant,depicted as an acetabular shell, made in accordance with an exemplaryembodiment of the present invention;

FIG. 2A is a fragmentary sectional view of a portion of the implant ofFIG. 1;

FIG. 2B is an enlarged schematic representation of a portion of theimplant of FIG. 2A;

FIG. 3 is an assembly view of the acetabular shell of FIG. 1 depictingalternative acetabular liners;

FIG. 4 depicts a graphical illustration of the relationship betweencontact stress and the thickness of a polymer material;

FIG. 5 depicts a graphical illustration of the stress/strain propertiesof different polymeric materials;

FIG. 6 depicts a graphical illustration of an exemplary finite elementmodel;

FIG. 7 depicts a graphical illustration of the stress/strain propertiesof porous metal;

FIG. 8 depicts a graphical illustration of the stress/strain propertiesof both conventional UHMWPE and antioxidant stabilized UHMWPE;

FIG. 9 depicts a graphical illustration of an unconfined compressionloading condition;

FIG. 10 depicts a graphical illustration of the effect of porosity on aconstruct's stiffness;

FIG. 11 depicts a graphical illustration of the effect of polymerthickness on a construct's stiffness;

FIG. 12 depicts a graphical illustration of stress/strain properties forvarious polymer thicknesses;

FIG. 13 depicts a graphical illustration of the stress/strain propertiesfor different thicknesses of the interdigitation layer between thepolymer and porous layers;

FIG. 14 depicts a graphical illustration of stiffness properties forvarious materials;

FIG. 15 depicts a graphical illustration of the oxidative index ofUHMWPE blend with 0.50 weight percent α-tocopherol acetate;

FIG. 16 depicts a graphical illustration of the trans-vinylene index ofan irradiated monoblock component; and

FIG. 17 is a fragmentary, cross-sectional view of an acetabular cupconstruct depicting exemplary thickness measurements of a UHMWPE layer.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate preferred embodiments of the invention and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

As shown in FIG. 1, an orthopedic component is depicted as acetabularshell 10. While described and depicted herein with specific reference toan acetabular shell, the orthopedic component of the present inventionmay be any orthopedic component, such as a glenoid component for use ina shoulder prosthesis system. Acetabular shell 10 has a substantiallyhemispherical shape and includes first, outer bone-contacting layer 12and second, inner layer 14 against which a corresponding femoralorthopedic component and/or natural femoral head articulate. In oneexemplary embodiment, second, inner layer 14 provides a surface againstwhich an additional acetabular liner (FIG. 3) may be seated.Additionally, acetabular prosthesis 10 may have any suitable shape knownin the art, such as hemispherical, and is generally configured to matewith a femoral head assembly, as indicated above, such as the femoralhead assembly disclosed in U.S. Pat. No. 7,306,629, entitled “FEMORALHEAD ASSEMBLY WITH VARIABLE OFFSET”, assigned to the assignee of thepresent invention, the entire disclosure of which is hereby expresslyincorporated by reference herein. Similarly, prosthesis 10 may have ashape similar to that disclosed in U.S. Pat. No. 5,879,398, entitled“ACETABULAR CUP”, assigned to the assignee of the present invention, theentire disclosure of which is hereby expressly incorporated by referenceherein.

In one exemplary embodiment, acetabular shell 10 has a stiffness whichis less than the stiffness of other, known acetabular shells.Specifically, acetabular shell 10 has an effective stiffness which issubstantially less than the effective stiffness of known acetabularshells. As used herein, absent an indication to the contrary or use of adifferent term, “effective stiffness” refers to the overall stiffness orelastic modulus of the entire construct of an orthopedic componentaccording to the experimental procedures set forth below in theExamples. As a result of the lower effective stiffness of the acetabularshell made in accordance with the present invention, the effect ofstress shielding on the natural acetabulum is lessened. As indicatedabove, stress shielding is a phenomenon in which the rigidity ofimplanted orthopedic components limits the transfer of forces to naturalbone stock that the bone stock would normally receive, such as duringthe loading of a joint. As a result, osteolysis may occur at the joint,which may result in bone resorption and weakening of the bone stocksurrounding the implanted orthopedic components. Thus, due to the lowereffective stiffness of acetabular shell 10, acetabular shell 10 allowsfor a greater amount of joint loading forces to be transferred to thenatural bone stock.

In one exemplary embodiment, bone-contacting layer 12 is formed from aporous material, such as a porous metal. In one exemplary embodiment,bone-contacting layer 12 is formed using Trabecular Metal™ technologyavailable from Zimmer, Inc., of Warsaw, Ind. Trabecular Metal™ is atrademark of Zimmer Technology, Inc. Such a material may be formed froma reticulated vitreous carbon foam substrate which is infiltrated andcoated with a biocompatible metal, such as tantalum, etc., by a chemicalvapor deposition (“CVD”) process in the manner disclosed in detail inU.S. Pat. No. 5,282,861, issued Feb. 1, 1994, entitled “OPEN CELLTANTALUM STRUCTURES FOR CANCELLOUS BONE IMPLANTS AND CELL AND TISSUERECEPTORS”, and in Levine, B. R., et al., “Experimental and ClinicalPerformance of Porous Tantalum in Orthopedic Surgery”, Biomaterials 27(2006) 4671-4681, the entirely disclosures of which are hereby expresslyincorporated by reference herein. In addition to tantalum, other metalssuch as niobium, or alloys of tantalum and niobium with one another orwith other metals may also be used.

Generally, with reference to FIGS. 2A and 2B, the porous tantalumstructure of bone-contacting layer 12 includes a large plurality ofligaments 16 defining open spaces, such as voids or channels 18,therebetween, with each ligament 16 generally including a carbon corecovered by a thin film of metal such as tantalum, for example. The openspaces between ligaments 16 form a matrix of continuous channels havingno dead ends, such that the growth of cancellous bone through the poroustantalum structure is uninhibited. The porous tantalum may include up to75%-85% or more void space therein. Thus, porous tantalum is alightweight, strong porous structure which is substantially uniform andconsistent in composition, and closely resembles the structure ofnatural cancellous bone, thereby providing a matrix into whichcancellous bone may grow to anchor acetabular shell 10 in thesurrounding bone of the pelvis.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an improved matrix for bonein growth and mineralization.

While described herein as being formed from Trabecular Metal™technology, first, bone-contacting layer 12 may be formed from anybiocompatible metal, such as Ti-6Al-4V, or a porous material, such as afiber metal. Additionally, in another exemplary embodiment, a nonporousmetal, such as titanium, is used to form bone-contacting layer 12 andthe inner surface thereof may be formed to incorporate a porous surface.The porous surface allows for inner layer 14 to be received within thepores of bone-contacting layer 12.

As indicated above, inner layer 14 is formed from a biocompatiblepolymer. In one exemplary embodiment, inner layer 14 is formed from aPAEK, such as PEEK. In another exemplary embodiment, inner layer 14 isformed from UHMWPE. In other exemplary embodiments, the biocompatiblepolymer may be a polyolefin, polyester, polyimide, polyamide,polyacrylate, and/or other suitable polymers. By utilizing a porousmetal, such as a metal manufactured using Trabecular Metal™ technology,and a biocompatible polymer, such as PEEK or UHMWPE, in combination, theelastic moduli of both materials forming acetabular shell 10 may beselected to be between the elastic moduli of cortical bone andtrabecular bone.

Specifically, the elastic modulus of porous tantalum made in accordancewith Trabecular Metal™ technology is approximately 3 GPa (gigapascal),the elastic modulus of PEEK is approximately 3.6 GPa, and the elasticmodulus of UHMWPE ranges from 0.5 GPa to 2.0 GPa, depending on thespecific UHMWPE used. Thus, various values within the range of 0.5 GPaand 2.0 GPa have been used for the elastic modulus of UHMWPE in thecalculations throughout the present application and the Examplescontained herein. The specific value for the elastic modulus of UHMWPEthat is used in any particular calculation is identified where relevant.In addition, the elastic moduli of cortical bone and trabecular bone are15 GPa and 0.1 GPa, respectively. In contrast, the elastic modulus ofCobalt-Chromium is 220 GPa and the elastic modulus of Ti-6Al-4V is 110GPa. Thus, the substantially low elastic moduli of porous tantalum madein accordance with Trabecular Metal™ technology and PEEK and/or UHMWPEprovide for the effective stiffness of acetabular shell 10 to besubstantially lower than the effective stiffness of known acetabularshells. For example, the effective stiffness of acetabular shell 10 maybe as low as 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 GPa or as high as1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 2.0, 2.5, 4.0, or 8.0 GPa. In oneexemplary embodiment, the effective stiffness of acetabular shell 10 isbetween 0.3 GPa and 1.5 GPa.

When UHMWPE is used as the polymer for inner layer 14, in order tobeneficially alter the material properties of UHMWPE and decrease itswear rate, the UHMWPE may be crosslinked. For example, UHMWPE may besubjected to electron beam or gamma irradiation, causing chain scissionsof the individual polyethylene molecules as well as the breaking of C—Hbonds to form free radicals on the polymer chains. The free radicals onadjacent polymer chains may then bond together to form crosslinkedUHMWPE.

In another exemplary embodiment, inner layer 14 is formed from anantioxidant stabilized polymer, such as antioxidant stabilized UHMWPE.In one exemplary embodiment, UHMWPE powder is combined with anantioxidant to form an antioxidant stabilized UHMWPE. By forming innerlayer 14 from an antioxidant stabilized polymer, some of the freeradicals in the polymer are quenched, which reduces oxidation and,correspondingly, increases the useful life of the polymer. For example,the UHMWPE may include an antioxidant such as Vitamin C, lycopene,honey, and/or tocopherol, i.e., Vitamin E. Additionally, as anytocopherol may be used, such as d-α-tocopherol, d,l-α-tocopherol, orα-tocopherol acetate, unless otherwise specifically stated herein, theterm “tocopherol” in its generic form refers to all tocopherols.Exemplary methods for combining UHMWPE with antioxidants are disclosedin co-pending U.S. patent application Ser. No. 12/100,894, entitled “ANANTIOXIDANT STABILIZED CROSSLINKED ULTRA-HIGH MOLECULAR WEIGHTPOLYETHYLENE FOR MEDICAL DEVICE APPLICATIONS”, filed Apr. 10, 2008,assigned to the assignee of the present invention, the entire disclosureof which is hereby expressly incorporated by reference herein.

In combining UHMWPE and tocopherol to form an antioxidant stabilizedUHMWPE, any mechanism and/or process achieving a substantiallyhomogenous blend of the components may be utilized. In one exemplaryembodiment, solvent blending is utilized. In solvent blending,tocopherol is mixed with a volatile solvent to lower the viscosity ofthe tocopherol and facilitate homogenous blending of the tocopherol withthe UHMWPE. Once the tocopherol is mixed with the solvent, thetocopherol/solvent mixture may be combined with the UHMWPE, such as witha cone mixer. The solvent is then evaporated, leaving only theantioxidant stabilized UHMWPE. In another exemplary embodiment,tocopherol may be blended with UHMWPE by precision coating oratomization. For example, tocopherol may be precision coated onto theUHMWPE powder using a MP-1 MULTI-PROCESSOR™ Fluid Bed connected to alaboratory module Precision Coater available from Niro Inc. of Columbia,Md. MULTI-PROCESSOR™ is a trademark of Niro Inc.

In another exemplary embodiment, low intensity mixing may be used. Lowintensity, i.e. low shear, mixing may be performed using a Diosna P100Granulator, available from Diosna GmbH of Osnabruck, Germany, asubsidiary of Multimixing S.A. In another exemplary embodiment, highshear mixing may be used. High shear mixing of UHMWPE and tocopherol maybe achieved using a RV02E or a R05T High Intensity Mixer, bothcommercially available from Eirich Machines of Gurnee, Ill.Alternatively, high shear mixing may be achieved using a ColletteULTIMAPRO™ 75 One Pot Processor available from Niro, Inc. of Columbia,Md. ULTIMAPRO™ is a trademark of Niro, Inc. Based on the results oftesting the above identified methods useful for combining UHMWPE andtocopherol, high shear mixing appears to provide favorable results,including an acceptable homogeneity and a low number of indications,i.e., areas of high tocopherol concentrations relative to thesurrounding areas as determined by visual inspection under ultravioletlight or by chemical measurements, such as infrared spectroscopy or gaschromatography. Additionally, in other exemplary embodiments, fluidizedbed, emulsion polymerization, electrostatic precipitation, wetting orcoating of particles, and/or master batch blending may be used tocombine the UHMWPE and tocopherol.

Irrespective of the method used to combine the UHMWPE and tocopherol toform the antioxidant stabilized UHMWPE, the components are combined inratios necessary to achieve a tocopherol concentration of between 0.01weight percent (wt. %) and 3 wt. %. In exemplary embodiments, thetocopherol concentration may be as low as 0.01 wt. %, 0.05 wt. %, and0.1 wt. %, or as high as 0.6 wt. %, 0.8 wt. %, and 1.0 wt. %, forexample. In determining the appropriate amount of tocopherol, twocompeting concerns exist. Specifically, the amount selected must be highenough to quench free radicals in the UHMWPE, but must also be lowenough to allow sufficient crosslinking so as to maintain acceptablewear properties of the antioxidant stabilized UHMWPE. In one exemplaryembodiment, a range of tocopherol from 0.1 to 0.6 wt. % is used tosuccessfully quench free radicals while still maintaining acceptablewear properties.

Once the antioxidant stabilized UHMWPE is substantially homogenouslyblended and the amount of tocopherol is determined to be within anacceptable range, the antioxidant stabilized UHMWPE is secured to bonecontacting layer 12 to form inner layer 14. In one exemplary embodiment,the antioxidant stabilized UHMWPE is secured to bone contacting layer12, as described in detail below. Once secured to bone contacting layer12, the antioxidant stabilized UHMWPE may be exposed to crosslinkingirradiation.

In preparing an antioxidant stabilized UHMWPE for exposure tocrosslinking irradiation, the antioxidant stabilized UHMWPE may bepreheated. In one exemplary embodiment, the antioxidant stabilizedUHMWPE may be preheated to any temperature between room temperature,approximately 23° C., up to the melting point of the antioxidantstabilized UHMWPE, approximately 140° C. In another exemplaryembodiment, the antioxidant stabilized UHMWPE is preheated to atemperature between 60° C. and 130° C. In other exemplary embodiments,the antioxidant stabilized UHMWPE may be heated to a temperature as lowas 60° C., 70° C., 80° C., 90° C., or 100° C. or as high as 110° C.,120° C., 130° C., 135° C., 140° C. By preheating the antioxidantstabilized UHMWPE before irradiation, the material properties of theresulting irradiated antioxidant stabilized UHMWPE are affected. Thus,the material properties for an antioxidant stabilized UHMWPE irradiatedat a relatively cold, e.g., approximately 40° C., temperature aresubstantially different than the material properties for an antioxidantstabilized UHMWPE irradiated at a relatively warm, e.g., approximately120° C. to approximately 140° C., temperature.

However, while the material properties of an antioxidant stabilizedUHMWPE irradiated at a lower temperature may be superior, the wearproperties, fatigue properties, oxidation level, and free radicalconcentration are all negatively affected. In contrast, whileirradiation of an antioxidant stabilized UHMWPE at a higher temperaturemay slightly diminish the material properties, it also results in ahigher crosslinking efficiency due to higher chain mobility andadiabatic melting. Additionally, by irradiating at a higher temperature,a greater number of crosslinks are formed. Thus, there are less freeradicals in the antioxidant stabilized UHMWPE and less tocopherol isconsumed by reacting with the free radicals during irradiation andimmediately thereafter. As a result, a greater amount of tocopherolremains in the blend that may react with free radicals during theantioxidant stabilized UHMWPE's lifecycle, i.e., after irradiation.This, in turn, increases the overall oxidative stability of theantioxidant stabilized UHMWPE.

Additionally, when the antioxidant stabilized UHMWPE forming inner layer14 and the porous metal of bone contacting layer 12 are irradiated, bonecontacting layer 12 may rapidly increase in temperature. Thus, thetemperature increase of bone contacting layer 12 should be taken intoaccount when determining the preheat temperature of the antioxidantstabilized UHMWPE forming inner layer 14 and the porous metal of bonecontacting layer 12.

After the temperature of the antioxidant stabilized UHMWPE forming innerlayer 14 reaches the desired preheat temperature, the antioxidantstabilized UHMWPE is subsequently irradiated to induce crosslinking ofthe UHMWPE. Thus, as used herein, “crosslinking irradiation” refers toexposing the antioxidant stabilized UHMWPE to ionizing irradiation toform free radicals which may later combine to form crosslinks. Theirradiation may be performed in air at atmospheric pressure, in a vacuumchamber at a pressure substantially less then atmospheric pressure, orin an inert environment, i.e., in an argon environment, for example. Theirradiation is, in one exemplary embodiment, electron beam irradiation.In another exemplary embodiment, the irradiation is gamma irradiation.In yet another exemplary embodiment, the crosslinking does not requireirradiation, but instead utilize silane crosslinking. In one exemplaryembodiment, crosslinking is induced by exposing the antioxidantstabilized UHMWPE to a total radiation dose between about 25 kGy and1,000 kGy. In another exemplary embodiment, crosslinking is induced byexposing the antioxidant stabilized UHMWPE to a total radiation dosebetween about 50 kGy and 250 kGy in air. These doses are higher thandoses commonly used to crosslink UHMWPE due to the presence ofantioxidant in the antioxidant stabilized UHMWPE. Specifically, theantioxidant reacts with some of the polyethylene chains that became freeradicals during irradiation. As a result, a higher irradiation dose mustbe administered to the antioxidant stabilized UHMWPE to achieve the samelevel of crosslinking that would occur at a lower dose in conventionalUHMWPE, i.e., UHMWPE absent an antioxidant.

Advantageously, by utilizing an antioxidant stabilized, crosslinkedpolymer to form inner layer 14 of acetabular component 10, the thicknessof inner layer 14 may be substantially reduced. For example, thethickness of bearing layer 26, shown in FIG. 2B and described below, ofinner layer 14 may be less then 4 mm and still maintain the same levelof contact stress as known acetabular cups having a bearing layerthicker than 4 mm. As discussed above, it was previously believed bypersons of ordinary skill in the art that bearing layer 26 of a metalbacked orthopedic component must be equal to or greater than 4 mm.However, acetabular cup prosthesis 10 has a substantially lower elasticmodulus than known acetabular cup designs. As a result, under the sameloading conditions, porous metal backed acetabular components willdeform more and provide more contact area so that the peak contactstress of acetabular cup prosthesis 10 is lowered, as shown in FIG. 4.Additionally, forming an acetabular component having a decreasedthickness of bearing layer 26 allows for a greater range of motion of amating femoral head component and also reduces the outer diameter of theacetabular component, which results in the preservation of a patient'snatural bone stock. Additional information regarding the specificproperties of an orthopedic component of the present invention having anantioxidant stabilized polymer inner layer 14 is set forth below in thecorresponding Examples.

To manufacture acetabular shell 10, bone-contacting layer 12 is formedusing traditional techniques, such as milling, molding, and/ormachining. Once formed and/or machined to have the desiredcharacteristics, inner layer 14 is attached to bone contacting layer 12.In one exemplary embodiment, inner layer 14 is attached to bonecontacting layer 12 by injection or compression molding. By utilizing aporous metal, e.g., a material formed using Trabecular Metal™ technologyor a metal having a porous coating to form bone-contacting layer 12, theattachment of the inner layer 14 allows the biocompatible polymerforming inner layer 14 to be received within the pores ofbone-contacting layer 12. This interaction creates a rigid, mechanicalbond between bone contacting layer 12 and inner layer 14 to fixedlyattach the two layers together, which allows for the transfer of theforces between bone contacting layer 12 and inner layer 14 that areencountered during the loading of the joint.

Referring to FIGS. 2A and 2B, the polymeric material of inner layer 14may be molded at least partially within the porous substrate of bonecontacting layer 12 to a desired depth to thereby form a unifiedconstruct by which the polymeric material of inner layer 14 is connectedto bone contacting layer 12 by interdigitation of the polymeric materialof inner layer 14 at least partially within the pores or channels 18 ofbone contacting layer 12. In this manner, referring to FIG. 2B, theimplant construct generally includes three layers, including porouslayer 22, which will contact and interface with bone tissue whenacetabular cup 10 is implanted within a patient, interdigitation layer24 in which the polymeric material of inner layer 14 is infiltratedwithin bone contacting layer 12, and bearing layer 26 comprising thepolymeric material of inner layer 14 and defining bearing surface 20.Thus, interdigitation layer 24 defines an intermeshing region in whichthe material of bearing layer 26 intermeshes with the material of porouslayer 22 and the interaction between the polymeric material of bearinglayer 26 and ligaments 16 defining voids or channels 18 of porous layer22 retains bearing layer 26 thereon.

By varying the thickness of each of these three layers, i.e., porouslayer 22, interdigitation layer 24, and bearing layer 26, as well as theporosity of porous layer 22, orthopedic components having varyingstiffness may be created. Advantageously, this allows for a surgeon orother medical professional to select an orthopedic component that has astiffness that is substantially similar to the bone stiffness of anindividual patient. Additionally, this allows for manufacturers oforthopedic components to design and manufacture orthopedic componentsthat have a stiffness that is substantially similar to the stiffness ofthe bone into which the orthopedic component is designed to beimplanted. For example, if the orthopedic component is an acetabularcomponent, the thickness of porous layer 22, interdigitation layer 24,and bearing layer 26 may be selected to create an orthopedic componenthaving an effective stiffness that is substantially similar to theelastic modulus of the bone of the pelvis. Similarly, if the orthopediccomponent is a glenoid component, the thickness of porous layer 22,interdigitation layer 24, and bearing layer 26 may be selected to createan orthopedic component having an effective stiffness that issubstantially similar to the elastic modulus of the bone of the glenoidof the scapula. Additional detailed information regarding specificthicknesses of each of the above-identified layers and the resultingeffective stiffness achieved using the same are set forth below in thecorresponding Examples.

Once the attachment, e.g., injection or compression molding, of innerlayer 14 to bone contacting layer 12 is finished, acetabular shell 10may be complete, i.e., ready for implantation. If acetabular shell 10 isnot finished at the conclusion of the attachment process, acetabularshell 10 may be further machined to create a final, implantableacetabular shell. In one exemplary embodiment, acetabular component 10,having bone contacting layer 12 and inner layer 14 forms the entireimplantable acetabular prosthesis. Thus, in this embodiment, acorresponding femoral prosthesis would articulate against bearingsurface 20. However, in another exemplary embodiment, bearing surface 20may define a contact and/or support surface for an acetabular liner. Inthis embodiment, in order to complete the acetabular component of a hipprosthesis system, an acetabular liner may be provided. The acetabularliner, such as liners 28, 30 described below, receive the head of afemoral prosthesis to replicate the natural hip joint.

To facilitate attachment of an acetabular liner to acetabular shell 10,an integral locking feature may be incorporated into inner layer 14during the process of attaching inner layer 14 to bone contacting layer12. Referring to FIG. 3, in one exemplary embodiment, groove 32 may beformed in inner layer 14 on bearing surface 20. For example, groove 32may extend around bearing surface 20 proximate to and/or at the equatorof inner layer 14. Groove 32 may be configured to mate with rib 34formed on acetabular liner 28, which may be formed from polyethylene.Acetabular liner 28 may then be retained within acetabular shell 10 viathe snap-fit connection of groove 32 and rib 34.

Additionally, in another exemplary embodiment, groove 32 may also beconfigured to mate with rib 36 formed on conversion ring 38. Conversionring 38 includes an inner tapered surface 40 which may be configured tomate with outer tapered surface 42 on acetabular liner 30. Acetabularliner 30 may be formed from metal or ceramic, for example. Thus,conversion ring 38 may be retained within acetabular shell 10 via thesnap-fit connection of groove 32 and rib 36. Acetabular liner 30 maythen be received by conversion ring 38 and retained within acetabularshell 10 by the locking engagement of inner tapered surface 40 ofconversion ring 38 and outer tapered surface 42 of acetabular liner 30.Additional details regarding the operation and use of conversion ring 28are set forth in co-pending U.S. patent application Ser. No. 11/401,727,entitled “ACETABULAR CUP CONVERSION RING”, filed Apr. 11, 2006, theentire disclosure of which is hereby expressly incorporated by referenceherein.

In another exemplary embodiment, the integral locking feature may beconfigured to receive a spring ring or any other locking component. Inyet another exemplary embodiment, the integral locking feature may be aMorse taper. This Morse taper may be formed as a self-locking taperconfigured to mate with a corresponding acetabular liner without the useof conversion ring 38. Advantageously, the use of the integral lockingfeature allows for a surgeon to easily switch between different linersand may also lessen the number of components needed to form the completeacetabular cup assembly.

Additionally, when inner layer 14 is attached to bone contacting layer12, inner layer 14 may be formed with smooth bearing surface 20 that maybe smoother than the inner or bearing surface of known acetabular cups.For example, known acetabular cups have a bearing surface that has anarithmetical mean roughness (Ra) of no less than 13 microns. Incontrast, smooth bearing surface 20 may have an Ra as low as 1, 2, 3, or4 microns or as high as 7, 8, 9, or microns. In one exemplaryembodiment, smooth bearing surface 20 has an Ra between approximately 4microns and 8 microns. The formation of smooth bearing surface 20 mayminimize wear debris that could potentially be created between bearingsurface 20 of inner layer 14 and outer surface 44 of liner 28 duringjoint articulation.

Further, if the attachment of inner layer 14 to bone contacting layer 12is accomplished by injection molding, surface 18 will have the samefinish, i.e., the same roughness, as the corresponding surface of theinjection mold. Thus, the corresponding injection mold surface can bepolished to a smooth finish and the need to further smooth or polishbearing surface 20 of acetabular shell 10 is substantially eliminated.Advantageously, this lowers manufacturing costs by shortening themanufacturing process and lessening the labor and tool costs of thesame. Additionally, the surface roughness of inner layer 14 may besubstantially similar to the surface roughness of liner 28. As a resultof the substantially similar surface roughness of inner layer 14 andliner 28, the creation of wear debris may be further minimized.

In preparation for the implantation of acetabular shell 10, a surgeonmay examine the acetabulum to determine the location having the greatestamount of and/or strongest area of bone stock. Referring to FIG. 1, oncethis determination is made, the surgeon may drill bone screw receivingapertures 46 within acetabular shell 10 that may align with thepreviously identified areas. Advantageously, by allowing a surgeon tocustomize the location of the bone screw receiving apertures in anorthopedic component, the costs of inventorying different orthopediccomponents with various bone screw receiving aperture configurations andmachining the same are substantially eliminated. Then, once acetabularshell 10 is ready for implantation, acetabular shell 10 is inserted intothe prepared, natural acetabulum and a bone screw, such as bone screw48, is positioned within aperture 46 and threaded into the patient'snatural bone stock. In one exemplary embodiment, once bone screw 48 isaffixed to the patient's bone stock, one of liners 28, 30 may beconnected to acetabular shell 10 in the manner described above. Inanother exemplary embodiment, bone screw 48 may be countersunk intoaperture 46, such the substantial entirety of head 50 of bone screw 48is positioned within outer layer 12. Alternatively, in another exemplaryembodiment, the liner may be attached to acetabular shell 10 prior toimplantation, and the two components implanted together and retained inposition in the patient's acetabulum using bone cement, for example.

Advantageously, due to the attachment of bone contacting layer 12 andinner layer 14, any dust and/or debris generated during the drilling ofbone screw receiving apertures 46 will be substantially contained withinthe polymeric debris of inner layer 14. This allows for a surgeon toeasily remove any dust and/or debris generated during the creation ofthe bone screw receiving apertures from acetabular shell 10 by removingthe metallic dust or debris and the larger polymer shavings that mayhave encapsulated additional metallic dust or debris during the drillingoperation.

EXAMPLES

The following non-limiting Examples illustrate various features andcharacteristics of the present invention, which is not to be construedas limited thereto. The following abbreviations are used throughout theExamples unless otherwise indicated.

TABLE 1 Abbreviations Abbreviation Full Word E_(eff) effective stiffnessPEEK polyetheretherketone R Radius mm millimeter H height Δ changeσ_(eff) effective stress ε_(eff) effective strain σ stress ε strainUHMWPE ultrahigh molecular weight polyethylene L length PM porousmaterial GPa Gigapascals lbf Pound feet π 3.14159265 r radialdisplacement t thickness T_(p) thickness of polymer layer T_(t)thickness of porous metal layer T_(i) thickness of interdigitation layerφ porosity kGy kilo Gray min minute MeV mega electron volt m meter °degrees C. Celsius FTIR Fourier Transform Infrared Spectroscopy wt. %weight percent MPa Megapascal UTS ultimate tensile strength YS yieldstrength HXPE highly crosslinked polyethylene OI Oxidation Index TTemperature DSC Differential Scanning Calorimetry ml milliliter nmnanometer TVI trans-vinylene index VEI d/l-α-tocopherol index Mc millioncycles molecular weight between crosslinks AVE aged vitamin E percentAVEI aged vitamin E index AV-OI aged oxidation index mg milligram cmcentimeter IR infrared VE % weight percent tocopherol Vol volume wt.weight VE tocopherol g gram DMA Dynamic Mechanical Analysis kJ kilojouleIzod Izod Impact Strength Conc. Concentration dm decimeter

Example 1 Effective Stiffness of PEEK Layer/Porous Layer Construct

An analytical model was used to study the effective stiffness of a layerof PEEK positioned adjacent to a layer of porous metal.

The effective stiffness of a layer of PEEK positioned adjacent to alayer of porous metal is dependent on both the material properties andgeometries of the individual layers. Thus, to attempt to remove anygeometric effects, an analytical model was created to include arectangular PEEK layer positioned atop a corresponding rectangularporous metal layer. The model incorporated frictionless contact betweenthe PEEK layer and porous metal layer and with none of the PEEKpenetrating into the pores of the porous metal layer. Additionally, theporous metal layer is assumed to have properties substantially similarto a porous metal layer formed in accordance with Trabecular Metal™technology, as described in detail above. The model was then subjectedto uniaxial compression. The stress is equally carried by both materialsand the compressive displacement of the materials is additive. Thus, thetotal displacement in the model is calculated as:

$\begin{matrix}{{\Delta\; L_{total}} = {{\Delta\; L_{PEEK}} + {\Delta\; L_{Porous}}}} \\{= {{ɛ_{PEEK}L_{PEEK}} + {ɛ_{Porous}L_{Porous}}}} \\{= {{\frac{\sigma}{E_{PEEK}}L_{PEEK}} + {\frac{\sigma}{E_{Porous}}L_{Porous}}}}\end{matrix}$

Thus, the effective stiffness of the two material construct is:

$\begin{matrix}{E_{eff} = \frac{\sigma}{ɛ_{total}}} \\{= \frac{\sigma\; L}{\Delta\; L_{total}}} \\{= \frac{L_{PEEK} + L_{Porous}}{{L_{PEEK}/E_{PEEK}} + {L_{Porous}/E_{Porous}}}}\end{matrix}$

Since different stiffness values are available for a porous metal layerformed in accordance with Trabecular Metal™ technology and sincedifferent applications may require different thicknesses of the PEEKlayer and the porous metal layer, the effective stiffness was modeledunder a range of values. Effective stiffnesses were modeled with thestiffness of the porous metal layer varying between 1-3 GPa, thethickness of the PEEK layer varying between 2.0 and 5.8 mm, and thethickness of the porous metal layer remaining constant at 4 mm. Theresulting effective stiffnesses are shown in TABLE 2, in addition tosome representative stiffnesses, i.e., Young's moduli, of variousmaterials.

TABLE 2

Referring to TABLE 2, the results indicated that with a porous metallayer of 4 mm and a PEEK layer of 2 mm, the effective stiffness of theconstruct was 1.3 GPa when 1.0 GPa was used as the elastic modulus ofthe porous metal layer. Additionally, with a porous metal layer of 4 mmand a PEEK layer of 2.0 mm, the effective stiffness of the construct was3.3 GPa when 3.0 GPa was used as the elastic modulus of the porous metallayer. Further, the results indicated that with a porous metal layer of4 mm and a PEEK layer of 5.8 mm, the effective stiffness of theconstruct was 1.8 GPa when 1.0 GPa was used as the elastic modulus ofthe porous metal layer. Additionally, with a porous metal layer of 4 mmand a PEEK layer of 5.8 mm, the effective stiffness of the construct was3.5 GPa when 3.0 GPa was used as the elastic modulus of the porous metallayer.

Example 2 Effects of Acetabular Cup Geometry on PEEK Layer/Porous LayerStiffness

A Finite Element (FE) model was used to study the effective stiffness ofan idealized acetabular hip cup model having a layer of PEEKinterdigitated with a layer of porous metal.

A 3-D parametric FE model was developed using commercially available FEsoftware ABAQUS 6.7 (ABAQUS, Inc., Providence R.I., USA). The FE modelincluded a femoral component, an acetabular cup, and a bone cementboundary. The femoral component was modeled to be rigid and sphericalwith a radius, R_(femoral), of 16 mm. The acetabular cup was modeled tohave a series of successive layers. The innermost layer is a UHMWPElayer which forms the acetabular liner of the acetabular construct andagainst which the femoral component articulates. The acetabular shell ofthe acetabular construct was modeled to have an innermost PEEK layer, aninterdigitated PEEK/porous metal layer, and an outermost porous metallayer. The porous metal layer was modeled to have the properties of amaterial formed in accordance with Trabecular Metal™ technology, asdescribed in detail above.

Additionally, the total polymer thickness, i.e., the combined thicknessof the UHMWPE and PEEK layers, was modeled to be 7.8252 mm, with twoseparate distributions between the polymers tested. The firstdistribution was modeled with a UHMWPE layer thickness of 5.8252 mm anda PEEK layer thickness of 2.0 mm and the second distribution was modeledwith a UHMWPE layer thickness of 2.0252 mm and a PEEK layer thickness of5.8 mm. The PEEK/porous metal layer, i.e., the interdigitated layer, wasmodeled to have a constant thickness of 2.1248 mm and the porous metallayer was modeled to have a constant thickness of 2.077 mm. This entireconstruct was modeled to be embedded in a block of bone cement having anouter radius, R_(cement), of 48.027 mm and a height, H_(cement), of48.027 mm.

Each of the material interfaces described above was also modeled to bein a completely bonded state, including the interface between theacetabular shell and the bone cement, and the coefficient of frictionfor articulation of the femoral component on the UHMWPE layer wasmodeled to be 0.02. The elastic modulus that was modeled for each of thelayers is set forth in TABLE 3 below. All materials were modeled to havea Poisson's ratio of 0.3.

TABLE 3 Layer Elastic Modulus (GPa) UHMWPE 1.00 PEEK 4.00 PEEK/Porousmetal 6.85 Porous metal 3.00 Bone Cement 3.00

Using the model set forth above, the femoral head was loaded by applyinga 1700 lbf load to the femoral head at a 45 degree angle relative to thepolar axis of the acetabular cup construct, with the outer boundary ofthe bone cement fully constrained, as shown in FIG. 6. The resultingdisplacement of the femoral component was calculated. The effectivestiffness of the acetabular construct was calculated using the effectivestress and the effective strain. Thus, the effective stress wascalculated as the force applied to the femoral component, i.e., 1700lbf, divided by the internal surface area of the cup, i.e., 2πR²_(femoral). The effective strain was calculated as the radialdisplacement of the femoral component, i.e., Δr_(head), divided by thetotal thickness of the acetabular cup construct, i.e.,t_(cup)=t_(UHMWPE)+t_(PEEK)+t_(Porous/PEEK)+t_(Porous). Therefore, theeffective stiffness is

$E_{eff} = {\frac{\sigma_{eff}}{ɛ_{eff}} = \frac{{F/2}\pi\; R_{femoral}^{2}}{\Delta\;{r_{head}/t_{cup}}}}$

Utilizing the model described above, the effective stiffness of theacetabular construct having a UHMWPE layer thickness of 5.8252 mm and aPEEK layer thickness of 2.0 mm is E_(eff)=0.45 GPa and the effectivestiffness of the acetabular construct having a UHMWPE layer thickness of2.0252 mm and a PEEK layer thickness of 5.8 mm is E_(eff)=0.54 GPa.

Example 3 Effects of Layer Thickness on a Porous Metal/UHMWPE OrthopedicComponent

A 2D stochastic microstructural FE model was developed to represent anidealized acetabular hip cup having a layer of UHMWPE interdigitatedwith a layer of porous metal. The internal geometry was based on a 2Drandom Voronoi structure generated by a custom program in FORTRAN. Thisprogram also allows changing parametrically the thickness of the threelayers, i.e., the UHMWPE layer, the interdigitated layer, and the porousmetal layer, to create different designs. Tantalum microstructs, such asthose formed in a material created using Trabecular Metal™ technology,are portions of ligaments 16, shown in FIG. 2a , having a loose end anddefining the perimeter of the porous metal layer. These tantalummicrostructs were modeled as a bilinear elastic material, as shown inFIG. 7, whereas UHMWPE was modeled as a multilinear elastic material, asshown in FIG. 8. Additionally, the various properties of the UHMWPE thatwere used in the model correspond to properties of an antioxidantstabilized, crosslinked UHMWPE, as set forth herein. The elements usedin the FE model were PLANE183 which are eight-node quadratic elementswith quadratic displacement behavior and are well suited to modelirregular meshes in 2D. The FE models thus generated generally includedmore than 50,000 elements. Once the model was prepared, uniaxialcompression was simulated using an unconfined compression loadingcondition, as shown in FIG. 9. Unconfined compression permits freedimensional change in directions transverse to the one in which load isapplied. FIG. 9 depicts a change in dimension δ₁ in the horizontaldimension parallel to the x-axis. The object is free to deform in thevertical dimension parallel to the y-axis.

Using the above-identified assumptions, a three-parameter, two-levelfull factorial analysis was performed to investigate the effect ofdesign parameters, i.e., the porosity (φ) of the porous metal, thethickness of the interdigitation layer (T_(i)), and the ratio of theUHMWPE layer thickness to the porous metal layer thickness(T_(p)/T_(t))) on the overall linear elastic stiffness, i.e., modulus,of the construct. The factors and their respective low and high levelsare shown in TABLE 4 below. The overall construct thickness was keptconstant with a total thickness of 10 mm. The porosity of the constructwas assumed to be 65 and 85%, respectively.

TABLE 4 Factor Low Level High Level Porosity, % 65 85 T_(p)/T_(t) 0.63.0 T_(i) [mm] 2 3

After determining the more significant effects from a Design ofExperiments (“DOE”) factorial analysis, a parametric study was performedto evaluate the mechanical behavior, such as the elastic modulus, of themodeled porous metal/UHMWPE construct. The DOE analysis determined whichof the parameters identified in TABLE 4 have the greatest influence onpredicted mechanical response (i.e., stiffness which defines the amountof deflection for a given load). Those parameters are % porosity,polyethylene thickness T_(p) and Trabecular Metal thickness T_(t).Different and representative values of porosities and layer thicknesseswere chosen based on current implant designs. The overall thickness ofthe entire construct and that of the interdigitation layer were keptconstant at 10 mm and 2 mm, respectively. Additionally, values of 0.6and 3.0 were used for the ratio of the thickness of the porous metallayer to the thickness of the polymer layer and values of 65% and 85%were used for the porosity of the porous metal layer. The matrix ofthickness and porosity combinations investigated is shown below in TABLE5. FE analysis was performed with ANSYS 10 finite analysis software(ANSYS, Inc., Canonsburg, Pa., USA) to simulate uniaxial compressionloading condition as set forth in FIG. 9.

TABLE 5 Analysis Factor Combinations Porosity, % 65 70 75 78 85 T_(p)(mm) 3 4 5 6 3 4 5 6 3 4 5 6 3 4 5 6 3 4 5 6 T_(i) (mm) 2 2 2 2 2 2 2 22 2 2 2 2 2 2 2 2 2 2 2 T_(t) (mm) 5 4 3 2 5 4 3 2 5 4 3 2 5 4 3 2 5 4 32

The factorial analysis was performed for the response of the overalllinear stiffness, i.e., elastic modulus, evaluated under compressionloading conditions. The effects of overall response were evaluatedconsidering a 95% confidence level. As shown in TABLE 6, the factorialanalysis showed a significant interaction of the ratio of the porousmetal layer thickness to the polymer layer thickness with the porosityof the metal. The thickness of the interdigitation layer was found tohave little effect on the construct stiffness and was kept constant forthe rest of the study. Specifically, it was determined that thepercentage contribution of each significant factor and interaction tothe model was 49.7% for porosity, 32.6% for the ratio of the porousmetal layer thickness to the polymer layer thickness, 17.5% for theinteraction of the ratio of the porous metal layer thickness to thepolymer layer thickness with the porosity of the metal, and 0.27% forthe thickness of the interdigitation layer. The results are plotted inTABLE 6 and FIGS. 10 and 11.

TABLE 6 Tp/Tt Porosity of Elastic Test Ti (mm) Ratio Metal Layer (%)Modulus (GPa) 1 2 0.6 65 2.413 2 3 0.6 65 2.46 3 2 3 65 1.325 4 3 3 651.343 5 2 0.6 85 1.134 6 3 0.6 85 1.235 7 2 3 85 0.982 8 3 3 85 1.046

Based on these results, it was shown that the higher the porosity levelof the porous metal, the lower the overall linear elastic structuralstiffness, or elastic modulus, of the construct. It was also found thatin the linear region the increase of UHMWPE thickness produced adecrease in the overall linear stiffness, or elastic modulus. Theeffects of the thickness of the interdigitation layer and the polymerlayer on the nonlinear overall mechanical behavior were also explored.It can be seen from FIGS. 12 and 13 that in the linear region (e<0.02%),the increase of the UHMWPE layer thickness will decrease the elasticmodulus (as indicated by the slope of the stress vs. strain curves)while the interdigitation layer thickness has little effect on theelastic modulus. In the nonlinear region (e>0.02%), the increase of theinterdigitation layer increases the elastic modulus, while the increasein the polymer layer thickness has little change in the material.

Additionally, a clinically successful orthopedic component must have anappropriate stiffness to promote primary fixation, during the firstmonths post-operatively, and secondary fixation with a constructive bonemodeling/remodeling response. Bone resorption and subsequent implantloosening may be caused by high bone stresses as well as low bonestresses due to “stress shielding” from an overly stiff implant. Basedon this, a preliminary design of a direct compression molded porousmetal, such as a metal made in accordance with Trabecular Metal™technology, and polymer, such as UHMWPE or PEEK, construct can be madeto have a stiffness that is substantially similar to the stiffness ofthe bone into which the construct is to be implanted. As shown by theresults of the current study, the construct's bulk linear stiffness isprincipally affected by the polymer layer thickness and the porosity ofthe porous metal. Additionally, the mechanical properties of bone arenonhomogeneous and anisotropic, varying between anatomical sites.Therefore, an implant may be designed with a given porosity of theporous metal layer and thicknesses of the polymer, interdigitated, andporous metal layers that targets the structural stiffness of its hostbone. In one exemplary embodiment, this can be achieved by varying thethickness of the polymer layer, as shown by the solid and dashed curvesin FIG. 14 with respect to UHMWPE.

Example 4 Effects of Antioxidant Stabilizing on Layer Thickness

Using the previous FE model developed in Example 3 above, a porous metaland UHMWPE design using conventional UHMWPE, i.e., UHMWPE that has notbeen antioxidant stabilized, was compared with a porous metal and UHMWPEdesign using antioxidant stabilized UHMWPE, i.e., UHMWPE thatincorporates an antioxidant, such as Vitamin E. The porosity of theporous metal was assumed to be 82%. Both conventional UHMWPE andantioxidant stabilized UHMWPE were modeled as multi-linear materials, asshown in FIG. 5, which indicates that the antioxidant stabilized UHMWPEhas a lower elastic modulus but higher yield stress than theconventional UHMWPE. All models were simulated as under uniaxialcompression and the geometry of all models was the same. Additionally,in the models, the thickness of both the conventional UHMWPE and theantioxidant stabilized UHMWPE layers was 3.5 mm and the thickness of theporous metal layer was 3.0 mm.

The stresses at the maximum stress point for both the conventionalUHMWPE and the antioxidant stabilized UHMWPE were recorded during theloading process and the results are set forth in TABLE 7 below.Referring to TABLE 7, the results indicated that the overall responsehas less variation in the non-linear region and under compressionloading. Regardless of the compression loading condition, it was shownthat compressive stresses were up to 8% lower in the antioxidantstabilized UHMWPE than in the conventional UHMWPE. Additionally, shearstresses were up to 13% lower in the antioxidant stabilized UHMWPE thanin the conventional UHMWPE. This indicates that under the same loadcondition, a porous metal/UHMWPE construct utilizing an antioxidantstabilized UHMWPE may experience less overall stress than a similarconstruct utilizing conventional UHMWPE. Thus, in such a construct, theantioxidant stabilized UHMWPE may have equivalent or better mechanicalperformance than conventional UHMWPE.

TABLE 7 Von Mises Compression [MPa] loading time strain conventional VE0.00 0.0000 0.00 0.00 0.05 0.0025 5.20 4.47 0.10 0.0050 8.07 7.50 0.150.0075 10.74 9.33 0.20 0.0100 11.82 11.36 0.25 0.0125 12.93 12.63 0.300.0150 13.96 13.43 0.35 0.0175 14.91 14.16 0.40 0.0200 15.80 14.85 0.450.0225 16.56 15.51 0.50 0.0250 17.17 16.14 0.55 0.0275 17.75 16.74 0.600.0300 18.29 17.30 0.65 0.0325 18.74 17.78 0.70 0.0350 19.15 18.22 0.750.0375 19.51 18.62 0.80 0.0400 19.86 19.01 0.85 0.0425 20.22 19.39 0.900.0450 20.54 19.76 0.95 0.0475 20.82 20.08 1.00 0.0500 21.09 20.39

Example 5 Feasibility Study of α-Tocopherol Acetate

Throughout the various Examples 5-13 set forth below, irradiated UHMWPEblends (i.e., antioxidant stabilized, crosslinked UHMWPE) are used,which have been irradiated according to one of three differentirradiation methods. As used in the Examples below, the term “UHMWPEblend” refers to an antioxidant stabilized UHMWPE or, if used to referto an antioxidant stabilized UHMWPE after exposure to crosslinkingirradiation, an antioxidant stabilized, crosslinked UHMWPE. As set forthabove, differences in the irradiation conditions and techniques mayaffect the resulting material properties of the UHMWPE blend. Therefore,in order to properly analyze and compare the results set forth in theExamples and corresponding TABLES, each of the irradiated UHMWPE blendsused in the Examples below are identified, where relevant, as havingbeen irradiated according to the one of the methods set forth below inTABLE 8. Additionally, the electron beam source is calibrated byperforming dosimetry at low irradiation doses and then parametricallydetermining the activation of the electron beam source needed to achievehigher doses. As a result, at higher irradiation doses, differences mayexist between the actual dose and the parametrically determined dose,which may cause differences in the material properties of the irradiatedUHMWPE blends.

TABLE 8 Irradiation Methods Method A Method B Method C Dose Rate (kGy-30-75 16-25 75-240 m/min) Dose Level (kGy) 160-190 133-217 90-200Electron Beam 10 12 10 Energy (MeV) Method of Water AluminumRadiochromic Dosimetry Calorimeter Calorimeter Film

The feasibility of blending α-tocopherol acetate with UHMWPE wasinvestigated. α-tocopherol acetate was obtained from DSM NutritionalProducts, Ltd. of Geleen, Netherlands and medical grade UHMWPE powderGUR 1050 was obtained from Ticona, having North American headquarterslocated in Florence, Ky. Isopropanol was then added to the α-tocopherolacetate as a diluent and the α-tocopherol acetate was solvent blendedwith the UHMWPE powder. The blending continued until two differentUHMWPE/α-tocopherol acetate blends were obtained, one UHMWPE blendhaving 0.05 wt. % α-tocopherol acetate and the other UHMWPE blend having0.5 wt. % α-tocopherol acetate. Each of the UHMWPE blends were thencompression molded to form four one-inch-thick pucks. Two pucks of eachUHMWPE blend, i.e., two pucks of the UHMWPE blend having 0.05 wt. %α-tocopherol acetate and two pucks of the UHMWPE blend having 0.5 wt. %α-tocopherol acetate, were preheated to 120° C. in a Grieve convectionoven, available from The Grieve Corporation of Round Lake, Ill. Thepucks were held at 120° C. for 8 hours. After the expiration of 8 hours,the pucks were irradiated at 10 MeV, 50 kGy-m/min dose rate at 65 kGyand 100 kGy dose at Iotron Industries Canada Inc. located in PortCoquitlam, BC, Canada.

The remaining two pucks of each UHMWPE blend, i.e., two pucks of theUHMWPE blend having 0.05 wt. % α-tocopherol acetate and two pucks of theUHMWPE blend having 0.5 wt. % α-tocopherol acetate, were heated to 40°C. overnight. The next morning, the remaining two pucks of each UHMWPEblend were irradiated at 10 MeV, 50 kGy-m/min dose rate at 100 kGy doseat Iotron Industries Canada Inc. located in Port Coquitlam, BC, Canada.

After irradiation, all of the pucks were cut in half and a film was cutfrom the center of each puck. The films were then subjected to FTIRanalysis using a Bruker Optics FTIR Spectrometer, available from BrukerOptics of Billerica, Mass. Both halves of each puck were then machinedinto flat sheets approximately ⅛ inch thick. One half of the flat sheetswere immediately subjected to FTIR. The other half of the flat sheetswere then subjected to accelerated aging in accordance with the AmericanSociety for Testing and Materials (ASTM) Standard F-2003, StandardPractice for Accelerated Aging of Ultra-High Molecular WeightPolyethylene after Gamma Irradiation in Air. Tensile specimens formedfrom the flat sheets were subjected to accelerated aging and were thensubjected to FTIR analysis. The OI and wt. % of α-tocopherol acetatewere determined from the FTIR results, set forth in TABLE 9, below, andin FIG. 14. However, there were interference peaks in the FTIR resultsthat prevented measurement of OI for the 0.5 wt. %, 65 kGy, unagedsample.

TABLE 9 FTIR Results wt. % wt. % tocopherol, tocopherol Dose, kGyCondition OI meas. 0.50 100 Un-aged <0/<0 0.17/0.15 Aged 0.0323 0.150.50 65 Un-aged *interference* 0.14/0.15 Aged 0.0083 0.14 0.05 100Un-aged 0.0300/0.0948 0.01/0.00 Aged 0.0647 <0   0.05 65 Un-aged0.0376/0.0940 0.02/0.00 Aged 0.0647 <0  

The FTIR results revealed that the OI of the UHMWPE blend having 0.05wt. % α-tocopherol acetate was generally higher than the OI of theUHMWPE blend having 0.50 wt. % α-tocopherol acetate. This is believed tobe because these samples still contained α-tocopherol acetate afterirradiation. As a result, the α-tocopherol acetate was still availablein these samples to react with free radicals and reduce the oxidativedegradation of the UHMWPE blend. Additionally, the FTIR results showedthat virtually no α-tocopherol acetate was left after irradiation of theUHMWPE blend having 0.05 wt. % α-tocopherol acetate and that aboutone-third of the α-tocopherol acetate was left after irradiation of theUHMWPE blend having 0.5 wt. % α-tocopherol acetate. Further, as shown inTABLE 10 below, tensile properties were similar for both the UHMWPEblends that were subjected to accelerated aging and the UHMWPE blendsthat were not subjected to accelerated aging. Finally, the FTIR resultssuggested that the UHMWPE blends containing α-tocopherol acetate havesimilar stabilization properties, i.e., a similar ability to preventoxidative degeneration, as UHMWPE blends containing similarconcentration of d,l-α-tocopherol.

TABLE 10 Mechanical Properties wt. % α- tocopherol Dose, kGy Elonga-Yield, UTS, acetate (Temperature) Condition tion, % MPa MPa 0.50 100(40° C.) Un-aged 356.7 23.1 61.7 Aged 360.1 25.3 65.3 0.50  65 (120° C.)Un-aged 384.6 21.8 61.4 Aged 378.4 24.2 65.1 0.05 100 (40° C.) Un-aged342.9 21.5 56.7 Aged 288.6 25.2 61.0 0.05  65 (120° C.) Un-aged 352.421.4 57.4 Aged 287.7 25.2 59.6

Example 6 Chemical Properties of UHMWPE Blended with Tocopherol

The chemical properties of d/l-α-tocopherol mechanically blended with aUHMWPE powder which was slab molded into bars and electron beamirradiated were investigated. To perform this investigation, DesignExpert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis,Minn., was utilized to setup a modified fractional factorial Design ofExperiment (DOE). The DOE evaluated five different variables: UHMWPEresin type, wt. % of d/l-α-tocopherol, preheat temperature, dose rate,and irradiation dose.

GUR 1050 and GUR 1020 medical grade UHMWPE powders were obtained fromTicona, having North American headquarters in Florence, Ky.d/l-α-tocopherol was obtained from DSM Nutritional Products, Ltd. ofGeleen, Netherlands. The GUR 1050 and GUR 1020 were separatelymechanically blended with the d/l-α-tocopherol by low intensity blendingusing a Diosna P100 Granulator, available from Diosna GmbH of Osnabrück,Germany, a subsidiary of Multimixing S.A. Both the GUR 1050 and the GUR1020 resins were mixed with the d/l-α-tocopherol in several batches tocreate UHMWPE blends of both resin types having 0.2 wt. %, 0.5 wt. %,and 1.0 wt. % d/l-α-tocopherol. Each batch of blended material wascompression molded into a slab and cut into bars of various sizes. Eachof the resulting bars was then preheated by heating to a preheattemperature in a Grieve convection oven, available from The GrieveCorporation of Round Lake, Ill. The preheat temperature was selectedfrom 40° C., 100° C., 110° C. and 122.2° C., as set forth in TABLE 11below.

After being preheated, the UHMWPE blend bars were electron beamirradiated according to Method C, set forth in TABLE 8 above, at aselected dose rate until a selected total irradiation dose wasadministered. The dose rate was selected from 75 kGy-m/min, 155kGy-m/min, and 240 kGy-m/min and the total irradiation dose was selectedfrom 90 kGy, 120 kGy, 150 kGy, and 200 kGy. The portion of each bar wasthen microtomed into 200 micron thick films. These films were thensubjected to FTIR analysis on a Bruker Optics FTIR spectrometer,available from Bruker Optics of Billerica, Mass. The FTIR results wereanalyzed to determine the VEI, wt. % d/l-α-tocopherol, the OI, and theTVI. The VEI and wt. % d/l-α-tocopherol were determined by calculatingthe ratio of the area under the d/l-α-tocopherol peak at 1275-1245 cm⁻¹on the resulting FTIR chart to the area under the polyethylene peak at1392-1330 cm⁻¹ and at 1985-1850 cm⁻¹. The OI was determined bycalculating the ratio of the area under the carbonyl peak on the FTIRchart at 1765-1680 cm⁻¹ to the area of the polyethylene peak at1392-1330 cm⁻¹. The TVI was determined by calculating the ratio of thearea on the FTIR chart under the vinyl peak at 980-947 cm⁻¹ to the areaunder the polyethylene peak at 1392-1330 cm⁻¹.

After the initial VEI, wt. % d/l-α-tocopherol and TVI were determinedfrom the FTIR analysis of the thin films, each of the thin films wereaccelerated aged according to ASTM Standard F-2003, Standard Practicefor Accelerated Aging of Ultra-High Molecular Weight Polyethylene afterGamma Irradiation in Air. The accelerated aged films were againsubjected to FTIR analysis on a Bruker Optics FTIR spectrometer,available from Bruker Optics of Billerica, Mass. The resulting FTIRcharts were analyzed to determine VEI, wt. % d/l-α-tocopherol, OI, andTVI according to the methods set forth above. Once subjected to FTIRanalysis, the aged files were placed in boiling hexane and allowed toremain there for 24 hours to extract the d/l-α-tocopherol. Afterextraction of the d/l-α-tocopherol, the aged films were again subjectedto FTIR analysis on the Bruker Optics FTIR spectrometer. The resultingFTIR chart was then analyzed to determine the OI in accordance with themethod set forth above. The additional FTIR analysis was performed toeliminate the d/l-α-tocopherol peak from interfering with the oxidationpeaks. An analysis of the results set forth in TABLE 11 below indicatethat selecting a warmer preheat temperature may result in a lower OI andmay also result in some of the d/l-α-tocopherol remaining in the UHMWPEafter irradiation.

TABLE 11 FTIR Results of Irradiated UHMWPE Blended with d/l-α-tocopherolPre-heat Dose VE level Dose Rate Resin Run (° C.) (kGy) (° C.)(kGy-m/min) Type (GUR) 1 122 150 1 75 1020 2 40 200 0.2 155 1020 3 12290 0.5 75 1020 4 122 200 0.2 155 1020 5 40 90 0.2 240 1050 6 122 90 0.275 1050 7 40 150 0.2 75 1050 8 122 150 0.2 75 1020 9 40 90 1 240 1050 1040 200 0.5 155 1020 11 122 90 0.2 240 1020 12 40 90 1 75 1020 13 40 1500.5 75 1020 14 122 150 0.2 240 1050 15 122 150 0.5 240 1020 16 40 1500.2 240 1020 17 40 90 0.2 75 1020 18 122 200 0.5 155 1020 19 122 90 1240 1020 20 40 150 1 240 1020 21 40 200 1 155 1020 22 122 200 1 155 102023 40 200 0.2 155 1050 24 122 200 0.2 155 1050 25 40 200 0.5 155 1050 26122 200 0.5 155 1050 27 40 200 1 155 1050 28 122 200 1 155 1050 29 40120 0.5 157.5 1050 30 122 120 0.5 157.5 1050 31 40 120 1 157.5 1050 32122 120 1 157.5 1050 33 40 90 1 75 1050 34 122 90 0.5 75 1050 35 40 1500.5 75 1050 36 122 150 1 75 1050 37 40 90 0.5 240 1050 38 122 90 1 2401050 39 40 150 1 240 1050 40 122 150 0.5 240 1050 VE % VE % VE Index VEIndex VE % 1370 1900 1370 1900 (aged) 1370 nm IR nm IR nm IR nm IR nm IRRun peak peak peak peak peak 1 0.803 0.682 0.046 0.171 0.493 2 0.0400.048 0.004 0.015 0.022 3 0.359 0.321 0.021 0.082 0.248 4 0.045 0.0540.004 0.016 0.037 5 0.047 0.055 0.004 0.016 0.063 6 0.061 0.071 0.0050.020 0.073 7 0.011 0.025 0.003 0.009 0.017 8 0.031 0.042 0.004 0.0130.033 9 0.194 0.165 0.012 0.044 0.272 10 0.731 0.626 0.042 0.157 0.54511 0.075 0.078 0.006 0.022 0.081 12 0.882 0.738 0.050 0.185 0.417 130.286 0.222 0.017 0.058 0.274 14 0.058 0.072 0.005 0.021 0.056 15 0.1620.151 0.011 0.040 0.279 16 0.051 0.053 0.005 0.016 0.050 17 0.078 0.0760.006 0.022 0.044 18 0.721 0.634 0.041 0.159 0.524 19 0.769 0.688 0.0440.173 0.430 20 0.781 0.597 0.044 0.150 0.531 21 0.769 0.591 0.044 0.1490.560 22 0.765 0.607 0.044 0.153 0.575 23 0.028 0.034 0.003 0.011 0.01624 0.051 0.053 0.005 0.016 0.041 25 0.288 0.249 0.018 0.064 0.281 260.320 0.282 0.019 0.073 0.309 27 0.284 0.222 0.017 0.058 0.281 28 0.3080.241 0.019 0.062 0.295 29 0.613 0.550 0.035 0.139 0.489 30 0.753 0.7000.043 0.176 0.445 31 0.283 0.240 0.017 0.062 0.279 32 0.306 0.288 0.0190.074 0.259 33 0.779 0.706 0.044 0.177 0.429 34 0.328 0.314 0.020 0.0800.209 35 0.143 0.125 0.010 0.034 0.247 36 0.803 0.758 0.046 0.190 0.44237 0.332 0.291 0.020 0.075 0.262 38 0.741 0.731 0.042 0.183 0.390 390.790 0.658 0.045 0.165 0.524 40 0.327 0.301 0.020 0.077 0.282 VE IndexOI VE % VE Index (aged) (Extrac- (aged) (aged) 1900 tion- TVI 1900 nm1370 nm nm IR aged) TVI (aged) Run IR peak IR peak peak FTIR FTIR FTIR 10.425 0.029 0.108 −0.010 0.061 0.062 2 0.033 0.003 0.011 0.078 0.0730.072 3 0.226 0.015 0.059 −0.008 0.046 0.052 4 0.048 0.004 0.015 0.0250.081 0.082 5 0.068 0.005 0.020 0.039 0.039 0.040 6 0.080 0.006 0.0230.000 0.054 0.048 7 0.029 0.003 0.010 0.086 0.068 0.064 8 0.042 0.0040.013 0.035 0.076 0.074 9 0.231 0.017 0.060 0.011 0.048 0.040 10 0.4680.032 0.118 0.005 0.076 0.077 11 0.084 0.006 0.024 0.007 0.055 0.054 120.359 0.025 0.091 −0.001 0.034 0.035 13 0.211 0.017 0.055 0.082 0.0750.074 14 0.070 0.005 0.020 0.001 0.072 0.072 15 0.244 0.017 0.063 0.0250.081 0.084 16 0.052 0.005 0.016 0.075 0.063 0.062 17 0.048 0.004 0.0150.098 0.045 0.045 18 0.458 0.030 0.116 −0.004 0.083 0.085 19 0.392 0.0250.100 −0.010 0.041 0.047 20 0.409 0.031 0.104 0.087 0.080 0.078 21 0.4290.032 0.109 0.061 0.081 0.087 22 0.457 0.033 0.116 −0.007 0.092 0.091 230.025 0.003 0.009 0.120 0.078 0.079 24 0.045 0.004 0.014 0.032 0.0840.085 25 0.241 0.017 0.062 0.009 0.075 0.073 26 0.268 0.019 0.069 −0.0020.085 0.083 27 0.220 0.017 0.057 0.024 0.080 0.079 28 0.229 0.018 0.0590.042 0.094 0.096 29 0.429 0.028 0.109 0.040 0.053 0.058 30 0.421 0.0260.107 0.000 0.063 0.064 31 0.236 0.017 0.061 0.063 0.061 0.061 32 0.2440.016 0.063 0.004 0.065 0.066 33 0.397 0.025 0.101 0.036 0.040 0.041 340.205 0.013 0.053 −0.005 0.049 0.051 35 0.211 0.015 0.055 0.067 0.0720.068 36 0.423 0.026 0.107 −0.012 0.055 0.058 37 0.234 0.016 0.061 0.0290.041 0.048 38 0.391 0.023 0.099 −0.004 0.038 0.042 39 0.440 0.030 0.1110.043 0.073 0.076 40 0.262 0.017 0.068 −0.004 0.068 0.068

Example 7 Mechanical Properties of UHMWPE Blended with d/l-α-Tocopherol

The mechanical properties of d/l-α-tocopherol mechanically blended witha UHMWPE powder which was slab molded into bars and electron beamirradiated were investigated. To perform this investigation, DesignExpert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis,Minn., was utilized to setup a modified fractional factorial Design ofExperiment (DOE). The DOE evaluated five different variables: UHMWPEresin type, weight percent of d/l-α-tocopherol, preheat temperature,dose rate, and irradiation dose.

GUR 1050 and GUR 1020 medical grade UHMWPE powders were obtained fromTicona, having North American headquarters in Florence, Ky.d/l-α-tocopherol was obtained from DSM Nutritional Products, Ltd. ofGeleen, Netherlands. The GUR 1050 and GUR 1020 were separatelymechanically blended with the d/l-α-tocopherol by low intensity blendingusing a Diosna P100 Granulator, available from Diosna GmbH of Osnabrück,Germany, a subsidiary of Multimixing S.A. Both the GUR 1050 and the GUR1020 resins were mixed with the d/l-α-tocopherol in several batches tocreate UHMWPE blends of both resin types having 0.2 wt. %, 0.5 wt. %,and 1.0 wt. % d/l-α-tocopherol. Each batch of blended material wascompression molded into a slab and cut into bars. Each of the resultingbars was then preheated by heating the bars in a Grieve convection oven,available from The Grieve Corporation of Round Lake, Ill., to a preheattemperature. The preheat temperature was selected from 40° C., 100° C.,110° C. and 122.2° C., as set forth in TABLE 12 below.

After being preheated, the UHMWPE blend bars were electron beamirradiated according to Method C, set forth in TABLE 8 above, at aselected dose rate until a selected total irradiation dose wasadministered. The dose rate was selected from 75 kGy-m/min, 155kGy-m/min, and 240 kGy-m/min and the total irradiation dose was selectedfrom 90 kGy, 120 kGy, 150 kGy, 200 kGy, and 250 kGy. Type V tensilespecimens, as defined by the American Society for Testing and Materials(ASTM) Standard D638, Standard Test Method for Tensile Properties ofPlastics, were machined from each of the UHMWPE blend bars. The Type Vtensile specimens were then subjected to ultimate tensile elongation,UTS, and YS testing in accordance with ASTM Standard D638. Izodspecimens were also machined from each of the UHMWPE blend bars andtested for izod impact strength according to ASTM Standard D256,Standard Test Methods for Determining the Izod Pendulum ImpactResistance of Plastics. Dynamic mechanical analysis (DMA) specimens werealso machined from each of the UHMWPE blend bars and tested using aModel DMA 2980 Dynamic Mechanical Analyzer from TA Instruments of NewCastle, Del.

An analysis of the results indicates that the total irradiation dose hadan influence on the izod impact strength, ultimate tensile elongation,and yield strength of the UHMWPE blends. Additionally, the preheattemperature had an influence on the ultimate tensile strength and yieldstrength. In contrast, the weight percent of d/l-α-tocopherol had aninfluence on ultimate tensile elongation and the dynamic mechanicalanalysis. Additional results from the testing are set forth below inTABLE 12, below.

TABLE 12 Mechanical Properties of UHMWPE Blended with d/l-α-tocopherolPreheat Dose VE Dose Rate Izod UTS YS DMA Std ° C. kGy Conc. kGy-m/minResin kJ/m{circumflex over ( )}2 Elongation % MPa MPa MPa 1 40 90 0.2 751020 90.79 Not Tested 5.45 2 122.2 90 0.2 75 1050 74.8 348.8 50.38 21.726.12 3 40 150 0.2 75 1050 59.66 300.9 56.2 25.1 6.78 4 100 150 0.2 751020 66.05 314.8 52.42 25.24 5.76 5 40 90 1 75 1020 111.19 Not Tested4.36 6 122.2 90 0.5 75 1020 91.55 Not Tested 5.1 7 40 150 0.5 75 102079.96 355.7 55.83 26.28 4.96 8 100 150 1 75 1020 81.25 Not Tested 4.78 940 90 0.2 240 1050 82.01 319.3 56.83 23.06 6.18 10 122.2 90 0.2 240 102084.5 Not Tested 5.43 11 40 150 0.2 240 1020 67.53 293.1 56.19 26.87 5.9612 100 150 0.2 240 1050 67.75 307.2 52.95 25.21 6.31 13 40 90 1 240 1050106.17 411.2 61.76 24.89 4.53 14 122.2 90 1 240 1020 94.66 Not Tested4.83 15 40 150 1 240 1020 93.79 Not Tested 5.6 16 100 150 0.5 240 102073.08 342.2 51.54 23.39 5.22 17 40 120 0.5 157.5 1050 99.87 374.6 57.9623.23 5.06 18 110 120 0.5 157.5 1050 90.67 363.6 50.97 22.15 5.42 19 40120 1 157.5 1050 94.34 352.5 58.5 23.64 5.53 20 110 120 1 157.5 105085.01 344.9 48.98 21.95 5.72 21 40 90 1 75 1050 107.07 396.4 61.25 23.135.02 22 122.2 90 0.5 75 1050 93.44 375.8 51.47 21.92 5.7 23 40 150 0.575 1050 82.09 330.4 56.65 25.78 4.62 24 100 150 1 75 1050 88.28 NotTested 5.4 25 40 90 0.5 240 1050 102.39 36.9 58.4 23.31 5.16 26 122.2 901 240 1050 96.6 381.9 50.3 21.29 5.44 27 40 150 1 240 1050 89.5 NotTested 5.16 28 100 150 0.5 240 1050 78.51 332.9 50.18 22.08 5.6 29 40200 0.2 155 1020 55.98 246.2 52.37 27.23 6.32 30 110 200 0.2 155 102052.98 268.4 48.28 24.82 6.05 31 40 200 0.5 155 1020 74.64 310.7 53.5325.42 5.38 32 110 200 0.5 155 1020 65.47 309.1 49.13 24.21 5.31 33 40200 1 155 1020 72.67 362.9 55.62 25.9 4.63 34 110 200 1 155 1020 66.62349.7 50.45 24.24 4.93 35 40 200 0.2 155 1050 57.82 226.4 50.92 25.37.15 36 110 200 0.2 155 1050 59.04 259.4 46.4 23.7 6.57 37 40 200 0.5155 1050 67.49 280.6 51.91 26.23 5.88 38 110 200 0.5 155 1050 64.6 304.851.23 25.14 5.7 39 40 200 1 155 1050 82.01 328.9 53.79 24.43 5.15 40 110200 1 155 1050 69.42 329.7 49.54 23.12 5.27 41 100 150 0.2 240 1050 NotTested 307.2 52.96 23.27 Not Tested 42 100 150 0.2 240 1020 Not Tested288.6 49.28 23.35 Not Tested

Example 8 Wear Properties of UHMWPE Mixed with d,l-α-tocopherol

The wear properties of UHMWPE mechanically blended with d/l-α-tocopheroland exposed to electron beam irradiation was investigated. To performthis investigation, Design Expert 6.0.10 software, obtained fromStat-Ease, Inc. Minneapolis, Minn., was utilized to setup a modifiedcentral composite Design of Experiment (DOE). The DOE evaluated fivedifferent variables: preheat temperature, dose rate, total doseadministered, d,l-α-tocopherol concentration, and cooling period, i.e.,the elapsed time from end of the preheat until initial exposure toirradiation.

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/l-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically mixed with the d/l-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/l-α-tocopherol in several batches tocreate UHMWPE blends having a selected wt. % of d/l-α-tocopherol. Thewt. % of d/l-α-tocopherol was selected from 0.14 wt. %, 0.19 wt. %, and0.24 wt. % d/l-α-tocopherol. Each of the blends were then consolidatedand formed into 2.5 inch diameter and 1 inch thick pucks. Each of theresulting pucks was then preheated by heating the pucks in a Grieveconvection oven, available from The Grieve Corporation of Round Lake,Ill., to a preheat temperature. The preheat temperature was selectedfrom 85° C., 100° C., and 115° C., as set forth in TABLE 13 below.

After being preheated, the UHMWPE blend pucks were then removed from theconvection oven for a cooling period. The cooling period was selectedfrom 7 minutes, 14 minutes, and 21 minutes, as set forth in TABLE 13below. The pucks were then electron beam irradiated according to MethodA, set forth in TABLE 8 above, at a selected dose rate until a selectedtotal irradiation dose was administered. The dose rate was selected from30 kGy-m/min, 52.5 kGy-m/min, and 75 kGy-m/min and the total irradiationdose was selected from 160 kGy, 175 kGy, and 190 kGy.

Pin-on-disc (POD) specimens in the form cylinders having a 9 mm diameterand 13 mm thickness were then machined from the UHMWPE blend pucks. Abidirectional pin-on-disc wear tester was then used to measure the wearrate of UHMWPE pins articulating against polished cobalt-chrome discslubricated by 100% bovine serum. These measurements were made inaccordance with the teachings of Bragdon, C. R., et al., in A newpin-on-disk wear testing method for simulating wear of polyethylene oncobalt-chrome alloy in total hip arthroplasty, published in the Journalof Arthroplasty, Vol. 16, Issue 5, 2001, on pages 658-65, the entiredisclosure of which is expressly incorporated by reference herein. Thebidirectional motion for the pin-on-disc wear tester was generated by acomputer controlled XY table, available from the Compumotor Division ofParker Hannifin of Cleveland, Ohio, which was programmed to move in a 10mm by 5 mm rectangular pattern. Affixed atop the XY table was a basincontaining six cobalt-chrome discs polished to an implant qualityfinish. The XY table and basin were mounted on a servo-hydraulic MTSmachine, available from MTS of Eden Prairie, Minn. The MTS machine thenloaded the UHMWPE blend pin specimens against the polished cobalt-chromediscs.

The MTS machine was programmed to produce a Paul-type curve insynchronization with the motion of the XY table. A Paul-type curve isexplained in detail in Forces Transmitted By Joints in the Human Body byJ. P. Paul and published by in the Proceedings Institution of MechanicalEngineers at Vol. 181, Part 37, pages 8-15, the entire disclosure ofwhich is expressly incorporated by reference herein. The peak load ofthe Paul-type loading curve corresponded to a peak contact pressure of6.5 MPa between each of the UHMWPE pin specimens and the cobalt-chromediscs. Tests were conducted at 2 Hz to a total of 1.128×10⁶ cycles.Analysis of the results indicated that the wear properties are affectedby both the concentration of d/l-α-tocopherol and the total irradiationdose. Specifically, the results indicated that increasing thed/l-α-tocopherol concentration increased the wear rate of the UHMWPEblends, while increasing the total irradiation dose decreased the wearrate of the UHMWPE blends. Additionally, the results indicated that bothdose rate and the cooling period had substantially no impact on the wearrate of the UHMWPE.

TABLE 13 Wear Properties of UHMWPE Mixed with d/l-α-tocopherol Dose RateOven to POD Preheat Dose VE (kGy- Beam Wear Run Block (° C.) (kGy) %m/min.) (minutes) (mg/Mc) 1 Block 1 85 190 0.11 30 7 0.96 2 Block 1 115190 0.11 30 7 1.14 3 Block 1 115 190 0.11 30 21 0.76 4 Block 1 85 1900.11 30 21 0.81 5 Block 1 115 160 0.11 30 7 1.86 6 Block 1 85 160 0.1130 7 1.37 7 Block 1 115 160 0.11 30 21 1.53 8 Block 1 85 160 0.11 30 211.57 9 Block 1 85 160 0.22 75 7 2.94 10 Block 1 115 160 0.22 75 7 2.1511 Block 1 85 160 0.22 75 21 2.41 12 Block 1 115 160 0.22 75 21 1.96 13Block 1 115 190 0.22 75 7 2.57 14 Block 1 85 190 0.22 75 7 1.87 15 Block1 115 190 0.22 75 21 1.87 16 Block 1 85 190 0.22 75 21 2.24 17 Block 1100 175 0.165 52.5 14 0.89 18 Block 1 100 175 0.165 52.5 14 1.18 19Block 1 100 175 0.165 52.5 14 1.24 20 Block 1 100 175 0.165 52.5 14 1.27

Example 9 Temperature Variations at the UHMWPE Blend/Substrate Interface

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/l-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically blended with the d/l-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/l-α-tocopherol to create a UHMWPE blendhaving 0.2 wt. % d/l-α-tocopherol.

A portion of the UHMWPE blend was then compression molded into a block.Another portion of the UHMWPE blend was compression molded into asubstrate to create a preform. The substrate was a 70 mm diameter porousmetal substrate in the form of a near-net shape acetabular shell. Theporous metal substrate was produced using Trabecular Metal™ technologygenerally available from Zimmer, Inc., of Warsaw, Ind., and described indetail above. This process was repeated to create five differentpreforms. The preforms were then individually heated to a preheattemperature in a Grieve convection oven, available from The GrieveCorporation of Round Lake, Ill. The preheat temperature was selectedfrom 100° C., 120° C., and 125° C. Once heated to the selected preheattemperature, the preforms were irradiated using Method B, set forth inTABLE 8 above, until a total irradiation dose was received. The totalirradiation dose was selected from 50 kGy, 75 kGy, and 150 kGy.Additionally, the UHMWPE block was heated to a preheat temperature of100° C. and irradiated using Method B until a total irradiation dose of150 kGy was received by the UHMWPE block.

The temperature of the preforms was measured at the UHMWPEblend/substrate interface, at a point in the UHMWPE blend adjacent tothe UHMWPE blend/substrate interface, and at a point in the center ofthe UHMWPE blend. Each of the temperature measures were taken using aType J thermocouple. Additionally, the temperature at the center of theUHMWPE blend block was also measured using a Type J thermocouple. Basedon the results, the presence of a porous substrate resulted in highertemperature readings in the UHMWPE blend. This is likely a result ofsubstrate reaching a higher maximum temperature than the UHMWPE duringirradiation.

Example 10 Effect of Substrate Orientation on UHMWPE Blend

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/l-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically blended with the d/l-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/l-α-tocopherol to create a UHMWPE blendhaving 0.5 wt. % d/l-α-tocopherol.

A portion of the UHMWPE blend was compression molded into a substrate tocreate a preform. The substrate was a 70 mm diameter porous metalsubstrate in the form of a near-net shape acetabular shell. The porousmetal substrate was produced using Trabecular Metal™ technologygenerally available from Zimmer, Inc., of Warsaw, Ind., and described indetail above. This process was repeated to create three differentpreforms. The preforms were then heated in a convection oven to apreheat temperature of 110° C. for a minimum of 12 hours. Two of thepreforms were then irradiated using Method A, as set forth in TABLE 8above, with the substrate of one of the preforms facing the irradiationsource and the substrate of the other preform facing away from theirradiation source. With the preforms in these positions, they wereexposed to a first, 100 kGy dose of irradiation. The preforms were thenallowed to sit in ambient air for 20 minutes. After the expiration of 20minutes, the preforms were exposed to a second, 100 kGy dose ofirradiation, for a total irradiation dose of 200 kGy.

The remaining preform was irradiated using Method B, as set forth inTABLE 8 above, with the substrate of the preform facing the irradiationsource. With the preform in this position, the preform was exposed to afirst, 100 kGy dose of irradiation. The preform was then placed in aconvection oven which maintained a constant temperature of 110° C. Afterthe expiration of four hours, the preform was removed from theconvection oven and exposed to a second, 100 kGy dose of irradiation,for a total irradiation dose of 200 kGy.

Each of the preforms was then cut through the center and the substrateremoved. The UHMWPE blend was then microtomed and subjected to FTIRanalysis using a Bruker FTIR Spectrometer, available from Bruker Opticsof Billerica, Mass., to determine the TVI of the UHMWPE blend. Thisanalysis was performed on the thickest part of the specimens. A sampleof the UHMWPE blend was then subjected to DSC using a TA InstrumentsQ1000, available from TA Instruments of New Castle, Del., to determinethe percent crystallinity of the UHMWPE blend. This analysis wasrepeated for samples of the UHMWPE blend taken from different locations.

In both of the monoblocks that were irradiated with the substrate facingthe irradiation source, a band of discoloration, i.e., translucence, canbe seen along the edge of the UHMWPE blend that interfaced with thesubstrate. As shown in FIG. 16, the FTIR analysis showed a substantialdecline in the TVI of the UHMWPE blend at a point just past theinterface between the UHMWPE blend and the substrate. Additionally, thepercent crystallinity at a point in the center of the UHMWPE blend wasapproximately 59%. The percent crystallinity decreased as the UHMWPEblend approached the interface with the substrate, with the percentcrystallinity reaching 48% in the translucent region near the UHMWPEblend/substrate interface, as shown in TABLE 14 below. In the preformthat was irradiated with the substrate facing away from the irradiationsource, the TVI of the UHMWPE blend was substantially more uniformthroughout the UHMWPE blend and the percent crystallinity varied by only2.2%. This may be a result of more uniform crosslinking occurring in thepreform in which the substrate faced away from the irradiation sourceduring irradiation.

TABLE 14 Percent Crystallinity of UHMWPE Blend % Crystallinity at %Crystallinity at the the center of the UHMWPE Blend/Substrate SpecimenUHMWPE Blend Interface Substrate Toward 59.29% 48.67% Irradiation SourceSubstrate Toward 58.60% 47.96% Irradiation Source Substrate Away from59.88% 57.66% Irradiation Source

Example 11 Effect of Irradiation Dose on UHMWPE Blend

Design Expert 6.0.10 software, obtained from Stat-Ease, Inc.Minneapolis, Minn., was utilized to setup a central composite responsesurface Design of Experiment (DOE). The DOE evaluated four differentvariables: d,l-α-tocopherol concentration, preheat temperature, totalirradiation dose administered, and irradiation dose per pass.

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/l-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically mixed with the d/l-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/l-α-tocopherol in several batches tocreate UHMWPE blends having a selected wt. % of d/l-α-tocopherol. Thewt. % of d/l-α-tocopherol was selected from 0.10 wt. %, 0.20 wt. %, 0.35wt. %, 0.50 wt. %, and 0.60 wt. % d/l-α-tocopherol. Each of the blendswas then compression molded into a substrate to create a preform. Thesubstrate was a 70 mm outer diameter porous metal substrate in the formof a near-net shape acetabular shell. The porous metal substrate wasproduced using Trabecular Metal™ technology generally available fromZimmer, Inc., of Warsaw, Ind., and described in detail above.

The resulting preforms were then placed inside a piece of expandablebraided polyethylene terephthalate sleeving and vacuum sealed inside analuminum-metallized plastic film pouch, such a pouch formed from apolyethylene terephthalate resin, such as Mylar®, which has been coatedwith a metal, such as aluminum, to reduce gas diffusion rates throughthe film. Mylar is a registered trademark of DuPont Teijin Films U.S.Limited Partnership of Wilmington, Del. The preforms remained in thiscondition until they were removed in preparation for exposing thepreforms to irradiation. Prior to irradiation, each of the resultingpreforms was preheated by heating the preforms in a Grieve convectionoven, available from The Grieve Corporation of Round Lake, Ill., to apreheat temperature, which was held for a minimum of 12 hours. Thepreheat temperature was selected from 60° C., 70° C., 85° C., 100° C.,and 110° C., as set forth in TABLE 15 below.

The preforms were then exposed to a selected total irradiation doseaccording to Method B, as set forth above in TABLE 8. The totalirradiation dose was selected from 133 kGy, 150 kGy, 175 kGy, 200 kGy,and 217 kGy. Additionally, the total irradiation dose was divided andadministered to the preforms in either two equal passes or three equalpasses, which are combined to achieve the total irradiation dose.Specifically, the preforms indicated to be “Block 1” in TABLE 15 belowreceived the total irradiation dose in two equal passes, while thepreforms indicated to be “Block 2” in TABLE 15 received the totalirradiation dose in three equal passes.

After irradiation, each of the UHMWPE blends was separated from thesubstrate and three Pin-on-Disc (POD) specimens in the shape ofcylinders having a 9 mm diameter and 13 mm thickness were then machinedfrom the UHMWPE blend pucks. A bidirectional pin-on-disc wear tester wasthen used to measure the wear rate of UHMWPE pins articulating againstpolished cobalt-chrome discs lubricated by 100% bovine serum. Thesemeasurements were made in accordance with the teachings of Bragdon, C.R., et al., in A new pin-on-disk wear testing method for simulating wearof polyethylene on cobalt-chrome alloy in total hip arthroplasty,published in the Journal of Arthroplasty, Vol. 16, Issue 5, 2001, onpages 658-65, the entire disclosure of which is expressly incorporatedby reference herein. The bidirectional motion for the pin-on-disc weartester was generated by a computer controlled XY table, available fromthe Compumotor Division of Parker Hannifin of Cleveland, Ohio, which wasprogrammed to move in a 10 mm by 5 mm rectangular pattern. Affixed atopthe XY table was a basin containing six cobalt-chrome discs polished toan implant quality finish. The XY table and basin were mounted on aservo-hydraulic MTS machine, available from MTS of Eden Prairie, Minn.The MTS machine then loaded the UHMWPE blend pin specimens against thepolished cobalt-chrome discs.

The MTS machine was programmed to produce a Paul-type curve [2] insynchronization with the motion of the XY table. A Paul-type curve isexplained in detail in Forces Transmitted By Joints in the Human Body byJ. P. Paul and published in the Proceedings Institution of MechanicalEngineers at Vol. 181, Part 37, pages 8-15, the entire disclosure ofwhich is expressly incorporated by reference herein. The peak load ofthe Paul-type loading curve corresponded to a peak contact pressure of6.5 MPa between each of the UHMWPE pin specimen and the cobalt-chromediscs. Tests were conducted at 2 Hz to a total of 1.128×10⁶ cycles.

The remaining portions of the UHMWPE blends were cut in half to formmicrotome films that were subjected to FTIR analysis utilizing a BrukerOptics FTIR Spectrometer, available from Bruker Optics of Billerica,Mass. The films were then accelerated aged according to ASTM StandardF2003, Standard Guide for Accelerated Aging of Ultra-High MolecularWeight Polyethylene. The OI of the post-aged films was then measured.

Once the measurements were taken, the post-aged films were placed inboiling hexane for 24 hours to extract any d/l-α-tocopherol remaining inthe films. The percentage of d/l-α-tocopherol extracted from the UHMWPEblend films was then determined. The remaining UHMWPE blend from themonoblock was then machined into 1/16″ flats and Type V tensilespecimens, as defined by ASTM Standard D638, Standard Test Method forTensile Properties of Plastics, were machined from the flats.

An analysis of the results, set forth below in TABLE 15, indicated thatwear increased with a lower total irradiation dose or with a higherconcentration of d/l-α-tocopherol. Additionally, the d/l-α-tocopherolconcentration had a significant impact on ultimate tensile elongation.The yield strength was affected the most by the preheat temperature,whereas UTS was affected the most by the total irradiation dose andd/l-α-tocopherol concentration. The OI was decreased with higher preheattemperatures and higher concentration of d/l-α-tocopherol. Although thepercentage of d/l-α-tocopherol decreased after irradiation and aging, asignificant amount of d/l-α-tocopherol still remained in the UHMWPEblend after irradiation and aging.

TABLE 15 Effect of Irradiation Dose on UHMWPE Blend Preheat Dose PODWear Run Block (° C.) (kGy) VE % (mg/Mc) 1 Block 1 100.00 200.00 0.201.01 2 Block 1 100.00 150.00 0.50 3.84 3 Block 1 100.00 150.00 0.20 1.594 Block 1 100.00 200.00 0.50 1.78 5 Block 2 59.77 175.00 0.35 1.97 6Block 1 70.00 150.00 0.20 1.76 7 Block 1 70.00 200.00 0.20 0.80 8 Block1 70.00 150.00 0.50 3.91 9 Block 1 70.00 200.00 0.50 2.38 10 Block 2110.23 175.00 0.35 2.05 11 Block 2 85.00 132.96 0.35 3.32 12 Block 285.00 175.00 0.60 2.34 13 Block 2 85.00 175.00 0.10 0.58 14 Block 285.00 175.00 0.35 2.28 15 Block 2 85.00 217.04 0.35 1.06 16 Block 185.00 175.00 0.35 1.94 17 Block 2 85.00 175.00 0.35 2.30 YS UTS VE % OIRun Elongation % (MPa) (MPa) (Aged) (Aged) 1 248.90 21.86 41.18 0.040.04 2 306.40 22.85 47.62 0.27 0.02 3 268.10 22.50 46.06 0.07 0.03 4293.00 22.03 43.03 0.25 0.02 5 261.80 24.45 49.27 0.12 0.08 6 248.1023.08 45.95 0.06 0.06 7 223.00 23.14 43.93 0.05 0.07 8 310.00 24.0451.23 0.25 0.03 9 272.30 23.86 48.34 0.24 0.02 10 273.20 23.76 46.950.17 0.04 11 288.90 23.92 49.37 0.17 0.04 12 289.60 24.37 49.24 0.290.04 13 213.20 23.21 45.01 −0.01 0.06 14 258.80 23.97 47.60 0.17 0.05 15234.00 24.41 45.00 0.13 0.06 16 269.70 23.39 48.64 0.14 0.02 17 264.1023.95 48.41 0.15 0.05

Example 12 Swell Ratio, Crosslink Density, and Molecular Weight BetweenCrosslinks

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/l-α-tocopherol wasobtained from DSM Nutritional Products, Ltd of Geleen, Netherlands. TheGUR 1050 was mechanically blended with the d/l-α-tocopherol using a HighIntensity Mixer, available from Eirich Machines of Gurnee, Ill. The GUR1050 resin was mixed with the d/l-α-tocopherol to create UHMWPE blendshaving 0.2, 0.5, or 1.0 weight percent d/l-α-tocopherol. The UHMWPEblends were then compression molded to form pucks that were thenmachined to form cubes having 5 mm sides. The UHMWPE cubes were thenheated to a preheat temperature selected from 40° C., 100° C., and 110°C. Once heated to the selected preheat temperature, the UHMWPE blendswere irradiated using Method C, set forth in TABLE 8 above, until atotal irradiation dose was received. The total irradiation dose wasselected from of 90 kGy, 120 kGy, 150 kGy, and 200 kGy.

The resulting UHMWPE blend cubes were then studied to investigate thepolymer network parameters of the UHMWPE blend by measuring thematerials' swell ratio (q_(s)) with a Swell Ratio Tester (SRT),Cambridge Polymer Group (Boston, Mass.), in accordance with ASTMF-2214-02. Knowing q_(s), the Flory interaction parameter (χ₁), themolar volume of the solvent (φ₁), and the specific volume of the solvent(v), the crosslink density (υ_(x)) and the molecular weight betweencrosslinks (M_(c)) of the material were calculated according thefollowing equations:

$v_{x} = {- \frac{{\ln( {1 - q_{s}^{- 1}} )} + q_{s}^{- 1} + {\chi_{1}q_{s}^{- 2}}}{\varphi_{1}( {q_{s}^{{- 1}/3} - {q_{s}^{- 1}/2}} )}}$$M_{c} = {\overset{\_}{v}v_{s}}$

Additionally, the swell ratio in stabilized o-xylene at 130° C. wasmeasured in the compression molded direction. The results of the testingare set forth in TABLE 16 below. For example, it was found that a UHMWPEblend having nominally 1.0% weight percent of d/l-α-tocopherol whenpreheated to nominally 40° C. and subsequently electron beam crosslinkedwith a total dose of nominally 200 kGy has a q_(s) less than about 4.3,a υ_(x) more than about 0.090 and a M_(c) less than about 11,142. It wasalso found that a UHMWPE blend having nominally 1.0% weight percent ofd/l-α-tocopherol when preheated to nominally 110° C. and subsequentlyelectron beam crosslinked with a total dose of nominally 200 kGy has aq_(s) less than about 3.6, a υ_(x) more than about 0.117 and a M_(c)less than about 8,577.

Also, it was found that a UHMWPE blend having nominally 0.5 wt. % weightpercent of d/l-α-tocopherol when preheated to nominally 40° C. andsubsequently electron beam crosslinked with a total dose of nominally200 kGy has a q_(s) less than about 3.8, a υ_(x) more than about 0.119and a M_(c) less than about 8,421. It was also found that a UHMWPE blendhaving nominally 0.5% weight percent of d/l-α-tocopherol when preheatedto nominally 110° C. and subsequently electron beam crosslinked with atotal dose of nominally 200 kGy has a q_(s) less than about 3.6, a υ_(x)more than about 0.109 and a M_(c) less than about 9,166.

Further, it was found that a UHMWPE blend having nominally 0.2 wt. % ofd/l-α-tocopherol when preheated to nominally 40° C. and subsequentlyelectron beam crosslinked with a total dose of nominally 200 kGy has aq_(s) less than about 2.8, a υ_(x) more than about 0.187 and a M_(c)less than about 5,351. It was also found that the UHMWPE blend havingnominally 0.2 wt. % of d/l-α-tocopherol when preheated to nominally 110°C. and subsequently electron beam crosslinked with a total dose ofnominally 200 kGy has a q_(s) less than about 3.0, a υ_(x) more thanabout 0.164 and a M_(c) less than about 6,097.

Additionally, it was found that under some conditions the crosslinkedUHMWPE blend exhibited a crosslink density of less than 0.200 moles/dm³.Under other conditions, the crosslinked UHMWPE blend having at least 0.1wt. % antioxidant exhibited a crosslink density of less than 0.190moles/dm³. Further, under certain conditions, the crosslinked UHMWPEblend having at least 0.1 wt. % antioxidant exhibited a crosslinkdensity of more than 0.200 moles/dm³ and had a molecular weight betweencrosslinks of less than 11,200 daltons.

TABLE 16 Swell Ratio, Crosslink Density, and Molecular Weight BetweenCrosslinks DOSE MATE- SAMPLE PREHEAT RATE RIAL DOSE TEMP PERCENT (kGy-RUN TYPE (kGy) (° C.) VITAMIN E m/min) 1 GUR 1020 90 40 0.2 75.00 2 GUR1050 90 100 0.2 75.00 3 GUR 1050 150 40 0.2 75.00 4 GUR 1020 150 100 0.275.00 5 GUR 1020 90 40 1.0 75.00 6 GUR 1020 90 100 0.5 75.00 7 GUR 1020150 40 0.5 75.00 8 GUR 1020 150 100 1.0 75.00 9 GUR 1050 90 40 0.2240.00 10 GUR 1020 90 100 0.2 240.00 11 GUR 1020 150 40 0.2 240.00 12GUR 1050 150 100 0.2 240.00 13 GUR 1050 90 40 1.0 240.00 14 GUR 1020 90100 1.0 240.00 15 GUR 1020 150 40 1.0 240.00 16 GUR 1020 150 100 0.5240.00 17 GUR 1050 120 40 0.5 157.50 18 GUR 1050 120 100 1.0 157.50 19GUR 1050 120 40 1.0 157.50 20 GUR 1050 120 100 1.0 157.50 21 GUR 1050 9040 1.0 75.00 22 GUR 1050 90 100 0.5 75.00 23 GUR 1050 150 40 0.5 75.0024 GUR 1050 150 100 1.0 75.00 25 GUR 1050 90 40 0.5 240.00 26 GUR 105090 100 1.0 240.00 27 GUR 1050 150 40 1.0 240.00 28 GUR 1050 150 100 0.5240.00 29 GUR 1050 2 × 100 = 200 40 0.2 240.00 30 GUR 1050 2 × 100 = 200110 0.2 240.00 31 GUR 1050 2 × 100 = 200 40 1.0 240.00 32 GUR 1050 2 ×100 = 200 110 1.0 240.00 33 GUR 1050 2 × 100 = 200 40 0.5 240.00 34 GUR1050 2 × 100 = 200 110 0.5 240.00 POD WEAR SWELL Vx = Mc = mg/1M RATIOXLD MWbXL RUN CYCLES V/V0 = q(s) X moles/dm{circumflex over ( )}3Daltons 1 5.09 0.44 0.068 14747 2 3.40 0.49 0.129 7764 3 2.65 3.15 0.500.147 6812 4 1.13 4.61 0.45 0.079 12652 5 6.15 0.42 0.051 19720 6 5.520.43 0.060 16706 7 4.22 0.46 0.091 11019 8 5.52 0.43 0.060 16706 9 3.750.48 0.110 9126 10 4.49 0.45 0.082 12143 11 3.84 0.47 0.105 9483 12 0.173.23 0.50 0.141 7115 13 7.13 0.41 0.040 24747 14 4.47 0.45 0.083 1205915 5.69 0.43 0.057 17502 16 4.46 0.45 0.083 12017 17 4.50 0.45 0.08212185 18 3.74 0.48 0.110 9087 19 5.32 0.43 0.063 15785 20 3.33 0.500.133 7496 21 5.78 0.43 0.056 17929 22 4.43 0.45 0.084 11891 23 3.923.84 0.47 0.105 9483 24 3.88 0.47 0.104 9642 25 5.59 0.43 0.059 17033 264.52 0.45 0.082 12270 27 4.37 0.46 0.086 11640 28 1.63 3.65 0.48 0.1158733 29 0.02 2.76 0.53 0.187 5351 30 0.09 2.96 0.52 0.164 6097 31 1.464.25 0.46 0.090 11142 32 0.64 3.61 0.48 0.117 8577 33 3.57 0.48 0.1198421 34 3.76 0.48 0.109 9166

Example 13 Free Radical Concentrations in UHMWPE Blended withd/l-α-Tocopherol

The impact of mechanically blending d/l-α-tocopherol with UHMWPE powderon free radical concentration of electron beam irradiated UHMWPE blendmolded pucks was investigated. To perform this investigation, DesignExpert 6.0.10 software, obtained from Stat-Ease, Inc. Minneapolis,Minn., was utilized to setup a modified central composite Design ofExperiment (DOE). The DOE evaluated five factors: preheat temperature,dose rate, irradiation dose, d/l-α-tocopherol concentration, andpredetermined hold time, i.e., the time elapsed between removal of theUHMWPE blend from the oven until the initiation of electron beamirradiation.

GUR 1050 medical grade UHMWPE powder was obtained from Ticona, havingNorth American headquarters in Florence, Ky. d/l-α-tocopherol wasobtained from DSM Nutritional Products, Ltd. of Geleen, Netherlands. TheGUR 1050 UHMWPE power was mechanically blended with the d/l-α-tocopherolby high intensity blending using an Eirich Mixer, available from EirichMachines, Inc. of Gurnee, Ill. The GUR 1050 resin was mixed with thed/l-α-tocopherol in several batches to create UHMWPE blends havingbetween 0.14 and 0.24 wt. % d/l-α-tocopherol, as set forth below inTABLE 17.

Each of the UHMWPE blends were then compression molded into 2.5 inchdiameter and 1 inch thick pucks. Each of the resulting pucks was thenpreheated by heating in a Grieve convection oven, available from TheGrieve Corporation of Round Lake, Ill., to a preheat temperature. Thepreheat temperature was selected from between 85° C. and 115° C., as setforth in TABLE 17 below. The pucks were then removed from the convectionoven and held for a predetermined period of time ranging between 7minutes and 21 minutes, as set forth in TABLE 17. After the expirationof the predetermined hold time, the pucks were electron beam irradiatedutilizing Method A of TABLE 8. The pucks were irradiated at a dose rateselected from between 30 kGy-m/min and 75 kGy-m/min until a total doseselected from between 160 kGy and 190 kGy was administered, as set forthin TABLE 17. Cylindrical cores approximately 1 inch long were machinedfrom the pucks. The cylindrical cores were then analyzed using a BrukerEMX/EPR (electron paramagnetic resonance) spectrometer, which has adetection limit of 0.01×10¹⁵ spins/gram and is available from BrukerOptics of Billerica, Mass. The resulting analysis indicated that preheattemperature, percent d/l-α-tocopherol, and dose level were allsignificant factors in determining the resulting free radicalconcentration of the UHMWPE blend. Specifically, preheat temperature andd/l-α-tocopherol concentration had a negative correlation with the freeradical concentration, while the total dose had a positive correlationwith the free radical concentration.

TABLE 17 Free Radical Concentration of UHMWPE Blends After VariousProcessing Free Dose radicals Rate Oven to (spins/ Preheat Dose (kGy-Beam gram × Run Block (° C.) (kGy) VE % m/min.) (minutes) E10-16) 1Block 1 85 190 0.11 30 7 2.87 2 Block 1 115 190 0.11 30 7 1.09 3 Block 1115 190 0.11 30 21 2.01 4 Block 1 85 190 0.11 30 21 3.7 5 Block 1 115160 0.11 30 7 1.09 6 Block 1 85 160 0.11 30 7 2.55 7 Block 1 115 1600.11 30 21 1.5 8 Block 1 85 160 0.11 30 21 2.77 9 Block 1 85 160 0.22 757 2.37 10 Block 1 115 160 0.22 75 7 0.826 11 Block 1 85 160 0.22 75 212.46 12 Block 1 115 160 0.22 75 21 1.38 13 Block 1 115 190 0.22 75 70.786 14 Block 1 85 190 0.22 75 7 3.22 15 Block 1 115 190 0.22 75 211.28 16 Block 1 85 190 0.22 75 21 2.94 17 Block 1 100 175 0.165 52.5 142.46 18 Block 1 100 175 0.165 52.5 14 2.66 19 Block 1 100 175 0.165 52.514 2.98 20 Block 1 100 175 0.165 52.5 14 3.03

Example 14 Effect of UHMWPE Thickness on the Fatigue Behavior of MetalBacked Acetabular Cups

The effect of reducing the thickness of an antioxidant stabilized UHMWPElayer in a metal backed acetabular cup design was investigated.

In order to prepare the test specimens, three Natural Cup™ metalacetabular shells made in accordance with Trabecular Metal™ technology,commercially available from Zimmer, Inc., of Warsaw, Ind., having ashell size of 54 mm and a head size of 40 mm, were obtained. The metalacetabular shells were compression molded with an antioxidant stabilizedUHMWPE to form monoblock acetabular cups. The antioxidant stabilizedUHMWPE of the acetabular cups was then crosslinked by exposing theantioxidant stabilized UHMWPE to crosslinking irradiation. The inner orarticular surface formed by the antioxidant stabilized UHMWPE layer wasthen machined for receipt of a 40 mm femoral head. All of the testspecimens were then cleaned to remove particles from the machiningoperation.

The thickness of the antioxidant stabilized UHMWPE layer of anacetabular cup is dependent on the specific shell size used and thedesired femoral head size for a particular acetabular cup configuration.Additionally, the thickness of the antioxidant stabilized UHMWPE layeris dependent upon the position at which the thickness of the UHMWPElayer is measured. In order to facilitate a comparison of the testresults, several different thickness measurements were taken for each ofthe acetabular cups in the test. Specifically, for the purposes of thisExample, the “Minimum Thickness” is defined as the thickness of theantioxidant stabilized UHMWPE layer measured from a point on the metalshells that is just below the titanium ring, which is formed at theequator of the Natural Cup™ metal acetabular shells, to a line tangentto the radius of the articular surface formed by the antioxidantstabilized UHMWPE layer, as shown in FIG. 17 as “MINIMUM”. A secondmeasurement, referred to as the “Wall Thickness”, is defined, forpurposes of this Example, as the thickness of the antioxidant stabilizedUHMWPE layer measured from the inner edge of the titanium ring of theNatural Cup™ metal acetabular shells to the outermost point of thearticular surface formed by the antioxidant stabilized UHMWPE layer, asshown in FIG. 17 as “WALL”. In addition, the thickness of theantioxidant stabilized UHMWPE layer at the pole of the cup and at apoint that is 45° from the pole of the cup were also measured, as shownin FIG. 17 as “POLE” and “45”, respectively. Based on the results of themeasurements, the antioxidant stabilized UHMWPE layer had a minimumthickness of 3.66 mm, a Wall Thickness of 1.33 mm, a thickness at thepole of the cup of 4.49 mm, and a thickness at a point that is 45° fromthe pole of the cup of 4.05 mm.

Once prepared, each of the three acetabular cups was then potted in ablock of poly methyl meth acrylate bone cement in a position such thateach of the cups had an inclination angle of 60°. The 60° inclinationangle was selected to capture a clinically relevant, but clinicallysteep, abduction angle. The cup specimens were then placed in ade-ionized water bath maintained at a temperature of 37±1° C. andsubjected to uniaxial loading on a MTS 858 Mini Bionix uniaxial fatiguetest machine, commercially available from MTS Systems Corporation ofEden Prairie, Minn. The temperature of 37±1° C. is representative of thein vivo environment, which is appropriate due to thetemperature-dependency of the mechanical properties of the UHMWPE.

The uniaxial loading was applied as a cyclic compressive force using aCobalt-Chromium-Molybdenum femoral head component, with aminimum/maximum stress ratio of R=0.1. The tests were be performed at afrequency of 3 Hz, with the first cup subjected to a force of 1000 lbs,the second cup subjected to a force of 1350 lbs, and the third cupsubjected to a force of 1700 lbs, until reaching a stopping point of 5million cycles or fracture, whichever occurs first. Once the tests werecompleted, each of the three acetabular cups were visually inspected fordamage to the antioxidant stabilized UHMWPE layer and the porous metallayer.

In contrast to traditional crosslinked UHWMPE acetabular cups made witha thicker UHMWPE layer, such as a UHMWPE layer of at least 6 mm, theacetabular cups of the present investigation all survived the testing,i.e., the stopping point of 5 million cycles was reached, and nodeformation of the antioxidant stabilized UHMWPE layer or the porousmetal layer was observed. In contrast, traditional crosslinked UHMWPEacetabular cups made with a fiber metal shell and having a UHMWPE layerthat is not antioxidant stabilized and a thickness of approximately 6 mmhave a fatigue strength of approximately 1400 lbs., as measured in asubstantially similar manner as indicated above. Thus, the use of anantioxidant stabilized UHMWPE layer allows for the acetabular cups ofthe present invention to have a UHMWPE layer that has a thickness thatis substantially less than the UHMWPE layer thickness of conventionalacetabular cups, while also providing an increase in the fatiguestrength of the same.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A method of manufacturing an orthopedic componentfor implantation into a bone and defining an acetabular shell configuredfor use in a hip replacement surgery, the orthopedic component having aporous bone contacting layer, an interdigitation layer, and an innerlayer, the method comprising: determining an elastic modulus of thebone; selecting a thickness of at least one of the porous bonecontacting layer, the interdigitation layer, and the inner layer basedon the elastic modulus of the bone; and molding the inner layer to theporous bone contacting layer to form at least one of the porous bonecontacting layer, the interdigitation layer, and the inner layer to havethe selected thickness; wherein the inner layer is formed from anantioxidant stabilized crosslinked ultrahigh molecular weightpolyethylene having a thickness of less than six millimeters, the innerlayer being configured to receive a femoral component, and theantioxidant stabilized crosslinked ultrahigh molecular weightpolyethylene being prepared by a process that includes: combiningultrahigh molecular weight polyethylene with an antioxidant to form ablend; processing the blend to consolidate the blend, wherein theconsolidated blend has a melting point; preheating the consolidatedblend to a preheat temperature below the melting point of theconsolidated blend; and irradiating the preheated consolidated blendwhile maintaining the preheated consolidated blend at a temperaturebelow the melting point of the consolidated blend; wherein theantioxidant is substantially uniformly distributed throughout theantioxidant stabilized crosslinked ultrahigh molecular weightpolyethylene; and wherein the interdigitation layer is defined by adistance over which the antioxidant stabilized crosslinked ultrahighmolecular weight polyethylene of the inner layer infiltrates pores ofthe porous bone contacting layer.
 2. The method of claim 1, furthercomprising, after selecting a thickness, selecting a thickness ofanother of at least one of the porous bone contacting layer, theinterdigitation layer, and the inner layer based on the elastic modulusof the bone.
 3. The method of claim 1, wherein the porous bonecontacting layer comprises a porous metal.
 4. The method of claim 1,wherein the inner layer comprises a polymer.
 5. The method of claim 1,wherein the acetabular shell has an effective stiffness between 0.1 GPaand 15 GPa.
 6. The method of claim 5, wherein the acetabular shell hasan effective stiffness between 0.1 GPa and 10 GPa.
 7. The method ofclaim 6, wherein said acetabular shell has an effective stiffnessbetween 0.3 GPa and 1.5 GPa.
 8. The method of claim 1, wherein theporous bone contacting layer has an elastic modulus between 0.1 GPa and15 GPa and said antioxidant stabilized crosslinked ultrahigh molecularweight polyethylene has an elastic modulus between 0.1 GPa and 15 GPa.9. The method of claim 1, wherein the porous bone contacting layerincludes a metal.
 10. The method of claim 1, wherein said porous bonecontacting layer includes a reticulated vitreous carbon foam substratecoated with a biocompatible metal by chemical vapor deposition.
 11. Themethod of claim 1, further including forming a substantiallyhemispherical liner from a liner material, wherein the liner isconfigured for receipt within said acetabular shell.
 12. The method ofclaim 11, wherein the liner material includes ultrahigh molecular weightpolyethylene.
 13. The method of claim 1 wherein the inner layer furtherincludes an integral locking feature.
 14. The method of claim 13,wherein the integral locking feature includes a groove configured toreceive a snap ring or a spring ring.
 15. The method of claim 14,wherein the integral locking feature includes a Morse taper.
 16. Themethod of claim 11, wherein the liner and the shell are prevented fromsubstantial relative translation by an integral locking feature.
 17. Themethod of claim 1, wherein the process to prepare the antioxidantstabilized crosslinked ultrahigh molecular weight polyethylene includesinjection molding polyethylene to the porous bone contacting layer. 18.A method of manufacturing an acetabular component configured for use ina hip replacement surgery and implantation into a bone, the acetabularcomponent having a porous bone contacting layer configured to contactand interface with the bone when the acetabular component is implanted,an interdigitation layer, and an inner layer, the method comprising:determining an elastic modulus of the bone; selecting a thickness of atleast one of the porous bone contacting layer, the interdigitationlayer, and the inner layer based on the elastic modulus of the bone; andmolding the inner layer to the porous bone contacting layer to form atleast one of the porous bone contacting layer, the interdigitationlayer, and the inner layer to have the selected thickness; wherein theinner layer is formed from an antioxidant stabilized crosslinkedultrahigh molecular weight polyethylene having a thickness of less thanfour millimeters, the inner layer is configured to receive a femoralcomponent, and the antioxidant stabilized crosslinked ultrahighmolecular weight polyethylene being prepared by a process that includes:combining ultrahigh molecular weight polyethylene with tocopherol toform a blend having between 0.1 and 3.0 weight percent tocopherol;processing the blend to consolidate the blend, wherein the consolidatedblend has a melting point; preheating the consolidated blend to apreheat temperature below the melting point of the consolidated blend;and irradiating the preheated consolidated blend with a totalirradiation dose of between 90 kGy and 1000 kGy while maintaining thepreheated consolidated blend at a temperature below the melting point ofthe consolidated blend, wherein the consolidated blend is secured to theporous bone contacting layer prior to irradiation; wherein thetocopherol is substantially uniformly distributed throughout theantioxidant stabilized crosslinked ultrahigh molecular weightpolyethylene; wherein the porous bone contacting layer has a porosity ofat least 55 percent; wherein the interdigitation layer is defined by adistance over which the antioxidant stabilized crosslinked ultrahighmolecular weight polyethylene of the inner layer infiltrates pores ofthe porous bone contacting layer; and wherein the acetabular componenthas an effective stiffness of between 0.3 GPa and 1.5 GPa.
 19. Theacetabular component of claim 18, wherein the inner layer exhibits novisible deformations after undergoing 5 million cycles of uniaxiallading under at least 1,000 pounds of force.